Definitely go out somewhere and order a real drink at a real bar on the actual day. It's so much fun to do it legally for the first time! Think about what the first legal drink you want to order is.
For the weekend, I suggest a small "pre-gaming" party where you can dance to ridiculous music and take ridiculous pictures in ways that only really make sense in your early 20's. And then go out to a bar/club and be silly in public. :-)
As for party themes...if it's a big party and you have enough space, it's fun to do a variety of different theme drinks/snacks. Have margaritas and chips and salsa in one room, martinis with olives and chips/hummus in another room, beer and nachos in another, wine and cheese in another, etc. Otherwise for a small party just pick one theme in one place and stick with it!
And of course, there are the drinking games...not too easy to describe on the internet, but with cards we used to play Kings or Ass-hole, or without cards quarters, speed quarters, flip-cup, survivor flip-cup (losing team has to vote someone off every time), beirut (not beer-pong..actual beer pong is played with a paddle!), "the finger game" (not at all dirty, as it sounds!), or "zoom, schwartz, bafigliano". I learned a game a few years ago that was amazing that was sort of like quarters but with a ping pong ball and solo cups, and you got to knock (empty) cups away from people and yell at them--I can't remember the last time I has so much fun.