Hi Adam and Ken,
Yes, you can wear your sandals and Hawiian shirts on "casual" nights, usually with a pair of docker type pants.
A lot of cruisers swear by some of the specialty restaurants, you will have to be the judge of that, have heard good reviews of Chops, so you may want to give it a try.
The only days where the pool chairs are at a premium are the "sea" days where just about everyone want to get out by the pool and enjoy the sun....they do post a sign about not
saving the lounge chairs and I have seen this problem improving. Royal Caribbean now will leave you 2 pool towels in your cabin each evening to use the next day, only requirement is that you need to return it to your cabin in order to get clean ones for the next day, otherwise they state, they will charge you $20 for the towels. You'll find not that many chairs being saved by towels these days, wonder why?? If you really want a nice located chair, get up early and have breakfast, then get up there and relax...
You can check out and purchase shore excursions on the Royal CAribbean web-site and also fill in all your embarkment/credit card information....its great and it saves a lot of time when you embark... If you should see some shore excursion that you really want to do, go ahead and pre-book it. You will also be able to buy them through your cabin TV once you board, but sometimes the popular ones do fill up quickly.
Here is a link to that area on their web-site, all you need is your reservation number to get into there:
Have a great cruise on Navigator, I know you'll love it!