Roughly 7500-7600 years ago a meteorite landed right in the middle of Saaremaa island causing extensive damage to the local life at the time. It is assumed that a 5-10-km wide meteorite broke into parts and landed here creating a large crater of up to 110m in width and 22 metres in depth. Today Kaali Meteorite crater is filled with water but definitely gives you those pre-historic jitters.
Kuressaare Castle is definitely worth a visit while in Saaremaa. The large castle faces the sea at the southern end of the town, on an artificial island ringed by a moat. It's an impressive example of a medieval stone castle in the Baltic region that has remained intact through centuries. The castle grounds are open to the public at all times if you just want to have a scenic picnic.