There are a large number of hotels in Heidelberg which you can book through the (online) travel agency of your choice. Aswell as staying in Heidelberg, you can also choose to stay in nearby towns and cities such as Mannheim. There are fairly regular connections between Heidelberg and Mannheim both via the local train network and using trams. There are connections throughout the night on Friday and Saturday, but limited connections on Sunday night (last train around 1am on Monday morning).
If you're on a low budget you might consider the Youth Hostel Heidelberg International, which is located within walking distance of the SotM conference venue.
If you prefer to camp, there are a few camp sites not very far from Heidelberg.