Indeed, it was our first visit to San Pedro & for that matter Belize. We enjoyed it very much...
As for tips, I have a couple; I found nearly all the info available via searching this msg board to be reliable. Either do a search, or if you have a specific question, toss it out...
On tip re: water, the stuff out the tap really is pretty offensive (that bleach taste- blech!), & I can drink Miami city water w/o thinking it tastes bad... I suppose if I lived in SP I'd get used to it, but plan on getting gallons / liters as needed for drinking (a gallon ran $1.50US at the grocery). Though I made ice for my rum drinks and coffee using tap water, w/o reeling from the taste; perhaps the bleach gasses off during freezing / brewing?
As to Caye Caulker, I am conflicted... it is very laid back, and I tend to like to go do stuff, & like a bit of night life. I really liked SP for being able to walk up/down the beach to hear a band at night (just a personal preference). CC is a very small town, lacking the hustle & bustle in SP. It seems to attract the young / college age, budget backpacker type traveler mostly. At least that's who I saw in town. Many of them hang out getting sun at the split. OTOH, most any excursion you might want to do (snorkel, tour mainland, dive, fish) can be done from CC as easily as from SP... It seems to have several good places to eat, and at least a couple decent beach bars.
It's a fine destination for a day trip: go over, wander around, get lunch, wander some more, take a dip, have a beer. Using the water taxi, if you get bored, just take an earlier one back
Not sure if you're taking kids, but we found a dock in town that my 9yo loved. It had a square walkway at its end (open in the middle), w/ all the seagrass cleared out in the middle ... water about 4 feet deep, and steps into the water. A bunch of local kids were swimming here... like a salt water pool almost. I had to drag him out of there, cuz the snorkeling wasn't much good w/ all that commotion
neat spot, if you have a young'un...