SoSiWe

I have never used one on our previous cruises either....but I was curious about RCL  Thanks

My son has cochlear implants, which include external parts similar to amped-up hearing aids that have to be charged and electronically dried out each night.  They're wall-powered (ie, the USB charging hub won't help us with this), so we were able to request an extension cord to have additional plug options in the room.  And we have parents with CPAP issues, so the special needs department knows us well already.  

Most Mariner cabins have two 110v outlets (North American style) and one 220v power port (European style).
They can't add extra outlets in a room.
I bring a plug adapter that allows me to use the 220v power port as well as the 110v outlets.  

https://shop.cruisehabit.com/products/2-to-1-eu-to-us-power-adapter-gain-extra-outlets-in-your-stateroom?variant=37171684299
Note this doesn't change the voltage from 220v down to 110v it just makes the 220v appear on a North American style outlet.  Consequently anything you plug into it must be capable of accepting 220v.  Many but not all modern devices are ready to accept 220v.  Phone charges, CPAP machines, laptop chargers, etc.  Many (but not all) of them can be plugged into 220v outlets.  This can free up the remaining two North American outlets for other things.   
Before using an adapter like this, make sure your devices can accept 220v.

I recommend you study the Mariner deck plans before leaving home , follow this link: https://www.royalcaribbean.com/cruise-ships/mariner-of-the-seas/deck-plans/1828/02. That way you will know the layout of the ship and won't waste time looking for a specific place. We do this on every cruise ship before we leave home. Yes, it's fun to wander around on a cruise ship looking at everything, but, we prefer to wander around knowing where we are and where we are going. Also, once you get on board go to the dining room and see where you are seated. 

Thanks everyone for such great advice. The cpap info was especially helpful as I always forget about that. ??‍♀️ 
I will read the links you all provided!  Happy sailing!!

Yes lol.  Multitasking sometimes works against me. 

I think you meant to say that the walls are metal.  I sure hope the walls aren't magnetic !  
?

I was on Mariner for a 3-night sailing in November. 
My suggestions, do the SkyPad on Embarkation Day its a very little line. Also, make sure you go and take care of your waivers and what not if you plan on doing the SkyPad and Flowrider. We loved the Traditional dining set up the food was very good. We also LOVED Cafe Promenade, though after the shows it can get quite busy. Also if you are doing the Balloon Drop and 80s Party and also plan on seeing the nighttime show, get to the side of the promenade that the theater is on, going through the crowds were NUTSO! 
We also experienced $2 mimosas in the casino, if your not doing the drink package. Which was much better price than the $7/each in the dining room for breakfast. 
We tried both the dining room and the Windjammer for breakfast and lunch. We preferred the Windjammer for breakfast and the Main Dining Room for lunch. 
Let me know if you have any ship-specific questions @SoSiWe.
 

+1 on that (for my kids)
I never use lanyards.

If you don't care about the print design, the dollar store is a cheap place to get the lanyards.  Especially if you are getting it for a group of people.  Money better spent on experiences and drinks!  I know some TA may offer lanyards/luggage tag sleeves if you booked with them. 

Amazon?
Here's a good overview https://www.royalcaribbeanblog.com/2016/08/08/food-royal-caribbean-cruise
 

@JLMoran  although so far we haven’t required any special equipment, your post is the first I have seen to note that there are special accommodations made for CPAP users and those with similar health-related devices. Had no idea there was even a special email address. Great info! ??
Oh, and ditto on the water shoes. Essential! ?

Don't forget you can bring 2 bottles of wine on board per stateroom and leave all your troubles behind.!

You can also purchase them at most terminals or at the shops on board.  The only terminal that I don't recall seeing them at was Cape Liberty.
We always bring magnets.  Lots of papers come through your cabin throughout the week.  The walls are metal.  It's great to be able to put them up instead of them cluttering the counter and tables.
Also, we pack magnetic hooks.  Lots of them.  I find the cabins never have enough hooks to hang things.  

Hi @SoSiWe, welcome to the message boards!
As far as things to bring, I think a big one that many of us here take along is some kind of USB charging hub for our various devices (phones, tablets, cameras, etc.), as even the newest ships (outside of Quantum class) don't include any USB ports in the room's outlet complement. If you don't yet have one of these and want to get one, there's a unit made by Anker that I own and can highly recommend. It passes the safety requirements for use on a ship -- no surge protector, not a high current draw -- and it can charge up to 5 devices at once, one of them using higher speed / higher amperage charging for a faster battery top-off.
If you happen to require a CPAP when you sleep, you don't need to bring an extension cord or distilled water. You can email [email protected] and let them know your requirement, and they'll provide both to you at no charge.
Which ports are you going to? Some of them have rockier beaches (including Adrenaline Beach on Royal's private destination, Labadee) and a pair of water shoes would be a good thing to have for them.
That's really all I can think of after checking my usual packing list. Hope this helps!

On the lanyard front, if you do decide to get one, I HIGHLY recommend a detachable one like this so you can easily hand off your seapass card to people who need to take it to scan without having to take the whole thing off your neck. It’s one of those things that might not be the most fashionable in the world, but IMO the convenience factor far outweighs that.
 
  

Lanyards are not given. that is your choice to buy or use. If you do a little searching on this blog you will find a wealth of information here to answer all of your questions. There is almost no topic that has not been covered. Also check out the podcasts. They are fun and informative with lots of tips. Have a great time planning. That can be one of the best parts of the trip!!!!
 
Happy sailing and welcome!

I haven't been on a cruise since 1998 (and it was also RCL), so I can't speak from recent experience, but from what I've read while planning my cruise this past year, not everyone gets or uses a lanyard.  It seems a lot of people are moving towards cell phone wallets or cases that hold your SeaPass and other flat essentials.  Or when you say, "does everyone get a lanyard," if you mean "receive" a lanyard from RC, like at boarding, I don't believe that happens at all.  I haven't seen anyone mentioning that.
Dining on RCL has these options:
"Traditional Dining" in the main dining room where you can select the early or late seating for your entire cruise and usually have the same wait staff and table mates (if any) throughout the week "My Time Dining" in the MDR where you can show up whenever you want and request a table on a walk-in/stand-by basis, or you can also reserve specific times for each night of your cruise through the cruise planner, like booking activities.   Specialty dining are the non-complimentary restaurants where you either pay through a flat-rate cover charge, a la carte pricing, or through a dining package. The Do or Do Not Bring list is probably very similar for RCL as for other cruise lines.  The biggies on the do not bring list would be fire hazards like extension cords, surge protectors, clothes steamer/irons (hair curling & flat irons are OK), space heater, coffee maker, etc., and any form of alcohol with the exception of two 750ml bottles of wine or champagne allowed per stateroom.  You're also allowed to bring (I believe) 12 bottles of non-alcoholic beverages on board such as soda, juice and bottled water.
For more first timer tips, I'd recommend scanning through this link if you haven't already.  Have fun planning and getting hooked on RCL!!
https://www.royalcaribbeanblog.com/getting-started