Noone will think any less of you if choose to do nothing but sit on a beach and catch tan for your whooooole stay. It takes a while to adjust from (allegedly) normal life in the cities. But I'm sure you'll be itching to do something active after a couple of days of chilling.
I'm actually not *in* Belize right now (though I'll be arriving in San Pedro tomorrow - wahey!), but I've been doing research the last couple of weeks, and if you just want somewhere cheap/cheerful, then Ruby's might be good for you. I'm heading to Pedro's myself - partly cos they seem to have a lively crowd there, but also cos there's wi-fi, which I need for work.
A couple of other places you might consider are Martha's or the Conchshell.http://ambergriscaye.com/marthas/index.htmlhttp://ambergriscaye.com/conchshell/index.html
Are you a learner diver, or experienced? I can't vouch for any of the San Pedro dive shops, but if they're anything like those on Caye Caulker you should have no problem. Belize is one of the best places to (learn to) dive - the instructors are relaxed and there are so many beautiful fish and corals to see - and you'll be absolutely spoilt for choice of dive sites once you're qualified. Your only limits will be money, time and energy! The sites close to SP are very popular, then there are the ones further out (such as The Blue Hole at Lighthouse Reef, Turneffe, Glover, etc)
For a chill-down day, a trip to Caye Caulker is a must - where signs in the street will implore you to "go slow". Swimming or snorkelling at the 'Split' is de rigeur. You can take side trips to the mainland, do some other water sport (sailing, kitesurfing, etc) or just kick back and relax with a chilly Belikin and watch the sunset!
You made the right choice - I'm sure you'll have an amazing trip!
If you have any more questions, keep 'em coming...!