i went to caye caulker about five years ago, so much has probably changed. but back then i loved it - as i do utila!
my girlfriend and I are doing much the same as you in a couple of weeks. we have three weeks in total to get from san pedro sula to belize city. and we also plan on spending a full week on utila. after that we'll spend a bit of time in placencia, whish is supposed to be nice, and then go inland for a bit. finally we'll probably finish off with a fre days out on caye caulker. these plans may change, but that is roughly what i'd do with three weeks.
if you'd like to spend a bit of time inland, after diving a lot in utila, i cannot recommend tikal too much. these ruins are absolutely great, but how much time it requires out of belize, i'm not sure....
but if you "just" want the caribean vacation i'd probably suggest caye caulker - laid back, and from what i gather, the prices are still pretty reasonable.
remember, however, that the divng in belize is more expensive than in utila, so if the budget is real tight, get most of the diving out of the way there.
btw - tulum is pretty nice for a couple of days when you cross into mexico!
not too structured answer, but i hope it helps a bit!