I'd take a bike tour with an established company. It's a great way to spend a morning or afternoon, and you'll get to see parts of the city you wouldn't otherwise see, and learn stuff you wouldn't otherwise know. I took a half-day with the "yellow bike" company- the bikes are cr*p, coaster brake cruisers, and the ride won't be physically challenging, but I was looking for history and architecture rather than exercise. If I had more time, I'd probably rent a bike and a strong lock, but transportation isn't a problem in the city center.