Rajgir is of course famous for being the spot where the  Prajnaparamita sutras, including the Heart Sutra were taught. DharmaWiki has a great page on Vultures Peak.

I your are in Bodhgaya you will defiantly want to consider doing the Mahakala Caves, Rajgir and Nalanda day trip. Click here for the map of the route to all 3.

Of these 3 Vultures peak was the highlight for me, not only do you get to visit the exact spot where the Heart Sutra was taught but up the top there is a very nice monastery and and amazing view. Be prepared for a little walking to get to the top thought, it is quite a climb. When I was there last they had a cable car to take you up and down (I went down in it, was pretty cool), but I cannot confirm if it is still operational. If you are going to see only one thing (apart from the Mahabodhi Temple) while in Bodhgaya then I would defiantly suggest visiting Vultures Peak.