I'm on the otherside of the country Russ, but I can give you some basic guidlines regarding your cultural quesitons. You have to be 25 to rent a car - this may differ slightly from state-to-state, but I would guess at 19 you're pretty much reserved to being chauffered around.
21 is the legal age of drinking here. That's not to say you couldn't get into some places. It would depend on the place and bouncer at the door, or the bartender if they would let you in or drink. I got into my fair share of bars and clubs while underaged - and I look very young. Of course, I also had talented friends with nice laser printers who could make decent fake ID's
Unfortunately, at your age America
might seem a little boring as far as partying on the town is concerned. I have a friend in Stratford-upon-Avon however (who was also 19 at he time), who still had alot of fun in the Seattle area despite not having the freedoms he did back home. I would imagine your locales would be much more exciting still.
Good luck and enjoy your trip!
Regards,
- Ryan