It really depends on my mood, but on some days when I'm bored I'll take my laptop out to the shed and plug a microphone in and start recording music. I have the drum kit and guitar amps and all that set up in the shed, so I'll record a drum track, then record the bass track, then the guitar track and finally the vocals, and put them altogether to make a song.
On other boring days I'll just sit in my room and visit various forums and listen to music. I've got a list of bands that I want to get into but haven't got around to yet (sort of like a to-do list), so if I'm going through a phase where I don't feel like listening to any of the bands in my music library I'll go through that list and find out which bands are worth getting the albums of and which can be thrown out.