I don't know anything about tours in Boston other than bike club rides, and you'd certainly need to have a bike for one of those. I just checked the Belmont Wheelworks site, and they appear to have rentals at their Wheelworks Too store, which is just around the corner from the main store. It's probably worth a phone call to them to check it out. (Wheelworks is a pretty major bike shop located in Belmont, MA. There may be other shops around that rent bikes too - I just really don't know where to send you because I've never needed to rent a bike here!)
Here's a link to the rental information: http://www.wheelworks.com/repair.htm#Rentals.
Charles River Wheelmen is a big Boston area biking club that has rides every Saturday & Sunday. The July rides aren't on the web site yet since they only show the current and next months, but you should be able to see if there is a ride that appeals to you that weekend within the next couple of weeks.
Good luck, and if you have other questions - ask away! There are a number of Boston area riders on this board; maybe someone else has a better answer for you.
--- Denise