KWofPerth

The Opera House is walking distance from the Overseas Passenger Terminal at Circular Quay, so there's no problem about missing the ship due to traffic.

I don't know the procedure for disembarking on embarkation day, but if it were me, I'd simply do the tour first and then check in for the cruise.

My suggestion would be to set them loose and let them roam free. As newly minted adults (legally) they will appreciate the freedom and I'm sure they'll find things of interest to them. As parents, you get credit for not being overly protective, while safe in the knowledge that they're somewhere on the ship. Win-win.

For Quantum, unless you have a GS or above, there is a charge for North Star (USD 20 pp for the normal experience, USD 40 pp for the extended session) and reservations will sometimes open up in the Cruise Planner, but not always. If you choose to do it, reserve a session on the app or at the iPad kiosks onboard.

We were on Anthem in March/April and we booked on a sea day, Nassau to be exact since we knew that we were not getting off of the ship.  As it so happened we got on the ship early and noticed that North Star was open that day as well (it was not in the Cruise Planner) and they were taking anyone that walked up at the time, the line was maybe 15 people deep and we only had to wait MAYBE 10 minutes before we were on!  So my advice would be book for a day, but try and get to the port early enough to try and get on that first day! 

Thanks for the replies. Both look great from the pictures! Thank you for sharing. Sounds like our Nassau day may be a good time, or even CoCo Cay maybe at tail end of it.  Bet that's a neat view too.  I've heard reservations via cruise planner are hard to get so will see what comes up.
And thanks for the comment about the bumper cars.  Unfortunately we are going during our March break so there will probably be a ton of kids on board (and thus, even longer lines).  We will try and line up before it opens to at least lesson the wait. (Which is why I'm hoping it doesn't overlap with any reservations we make).
KWofPerth, hope your reservations opens up soon.

They can book really quickly when they are released on the Cruise Planner (Pixels was sold out within 24 hours for my Ovation sailing) I really don't believe it's the only opportunity to book them though - I think they hold back a certain amount for booking on the ship (plus there's likely stand by lines for no shows).

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

Thats a really good idea, like that

Though I would say the Number 1 item lost on a ship is my Waist line :P

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.

That's a genius tip!
I've never lost my phone, but I have lost my dignity on the FlowRider a couple of times.