Sackboy

Drink. Lots.

This is a non-food advice
1) Bring a small battery pack for your phone. Its nice to have so you don't run out of power for your cell phone
2) Bring magnetic hooks. They are nice cause you can hang your hats/lanyards/and other items. I have also used them to hang up a shoe rack organizer and use it to put common items eg sunglasses, sunscreen etc. 
3) bring a USB splitter such as this it allows for charging of multiple devices rather than having a plug for every single device. 
4) if you do buy a photo package via the cruise planner just know that once you choose a digital print you cannot un-choose it to get a different photo. So wait till the last night before picking your pictures
5) Ask a lot questions on this blog. There are many experienced Royal passengers on this forum that are willing to share their experience. 
6) Final advice is have print outs and don't just rely on your phone for digital copies. I don't know where the Mariner is sailing out of but when you sail out of Cape Port liberty in Bayonne NJ and have T-mobile as your cell phone provider, the second you step into the terminal you have no signal. So it is really hard to pull up your set sail pass on the app or communicate with other members of your party. So rather than leaving that to chance if you have a print out of all your documents you will have no issues. 
 

You can always visit Windjammer if you want a little more dessert (or other food). I always seem to want more veggies. No judging when on a cruise!
Another tip: You can ask your waiter at MDR for food  to take back to your room for later, and put it in your mini-fridge.  Thinking a dessert or fruit and cheese to go with your wine in your room.
Each room may bring up to 2 bottles of wine (750 ml each on the cruise). Your steward can bring you a corkscrew  and 2 glasses, and the Steward will replace the glasses with clean glasses after use. 
Finally, in the UK I bought a  USB port to charge my phone. This will come in handy because I can use more plugs in the room, with US and UK outlets.

Title talks about Covid restrictions, post mentions nothing about covid restrictions. 

Title talks about Covid restrictions, post mentions nothing about covid restrictions. 

Title talks about Covid restrictions, post mentions nothing about covid restrictions. 

Just an FYI, Johhny Rockets only does breakfast on Oasis class ships. Mariner isn't an oasis class, so they will most likely be open for lunch and early dinner. 
Also, you can ask for smaller portions of items to try. I do this when I want to try multiple items but not waste a ton of food. 

Order multiple items for any course you want (or all 3 courses). I or members of my traveling party have done this many times and the servers have always been happy to oblige!
Only other food tip is to take a quick trip through the Windjammer (buffet) if you like to try different stuff or eat a lot or both (like I do) even if you're eating in the MDR. The offerings change nightly and you never know when you might stumble upon something you find especially appealing. If nothing looks special, move on.

Izumi is "specialty dining" and costs more -- for sushi you'd look at their a la carte menu.  My son has tried it (he says it's OK but nothing to brag about.)
In the Main Dining Room (MDR) you can ask for multiple dishes, but please remember the burden it puts on the wait staff, if you make it a habbit.  However, they would be happy to order you something to try that you are unsure of along with something else.  There are members who do this regularly.  I do not -- too much food.   Sometimes a dish is not to your liking and you can ask your waiter to take it away and bring something else.  That is a service they always provide. 
If you are a foodie, consider a dining package of some sort.  Our Travel Agent gives us a couple of coupons when we reserve a balcony room. We found a lunch at Jamie's was outstanding (cheaper than buying a dinner but just as good), and we enjoyed our filet mignon at Chops on our recent Mariner cruise.
Our take on Johnny Rockets is that it was too crowded to really enjoy the experience.
If you download the Royal App to a smartphone, you will find that you can browse menus for each day.  We were able to do this the week or two before the cruise, although menu selections did change when we boarded.