The Big Bus Company (and some others) have "hop-on, hop off" deals. It's about $25 or so for a 24 hour pass, and it's a great way to get around. The initial tour is worth that cost alone. First, make sure your bus has a live guide, and sit near him/her. They're often hilarious, and banter with the nearby passengers in between "talks". Take the entire (90 minute or so) tour, where they give you all the highlights of the city, interesting historical tidbits, after which you can decide where you want to return to explore. Your pass is good for 24 hours of this "hopping on, hopping off". The buses are in abudance, run about every 10-15 minutes, so you're never waiting. It's a great deal, you get a little history lesson, a lot of laughs, a great view from the top deck, and you really make the most of a day in the city!

I've done this twice in the past few years, and will be doing it again in September!

Charlene