It is wonderful that you have fallen in love with La Isla Bonita. Independence Day in San Pedro is great fun. You'll want to see the parades, go to the block party and watch the fireworks. The organisers often to not advertise the details of the events until much closer to the date, but you'll hear about them here and on the local newspaper blogs you see links to on your right hand side on this page.
One thing you will see is that everyone proudly flies the Belize flag from their homes and golf carts. If there are full moon parties, you'll have to find out closer to the date. You'll find that in Belize that there is little sense of urgency about most things, so it is best not to try to plan in advance but just roll with the flow.
Sea Turtle, BTL, C's Phones and Cellular World are all on Middle Street and you should be able to purchase a sim card cheaply at one of those locations.
There are several sports bars here, Cholo's being one of the more popular. During big games, lots of bars show them on their TV's. Just ask around when you get here and you'll find out where the action will be for sure.
Drink driving is most definitely an offence. I am not sure about open drink containers though. I have seen the police stop tourists who were driving with bottles of beer in their hands, but I would think it is unlikely you would be stopped if the passenger is drinking. Just be sensible and respectful and you'll have no problems.
The nearest American Embassy is in Belmopan on the Belize mainland.
I hope you have a wonderful experience visiting Belize and that you'll want to come back again and again like most of our visitors.