I like to spend time by myself, just generally sitting about and playing a few games. If I'm really stressed I'll just go to bed for an afternoon and watch a TV series or something like Red Dwarf.
I have a sofa in my spare room that faces the window, which looks out over the Yorkshire countryside. So I just sit and stare out the window, maybe with BBC 7 on the hi-fi... aaaaaaah...
Go paint balling! It's like CounterStrike but 100 times as much fun, and you get some exercise. But if I'm not looking for an adrenaline rush, I like to just watch some DVDs.
Sit in the sun, reading a book (with SPF 100 sun cream on ) - perfect way to spend the afternoon, maybe sneak a beer in there too.
I play some of my older, better computer games - Deus Ex, Planescape Torment, Fallout. I also surf when I'm really stressed, but my last trip ended with a mate of mine getting lots of broken bones at a dirt bike comp, so I don't think I'll be going again for a while. Mainly though, I buy a bottle of expensive wine or whisky and I sit in with the missus, talking and eating seafood. I loves my seafoods.
Well i like to watch a good movie, but not in tv (the adds make me become crazy) or just listen to music while laying on the bed...
Never mind all this nonsense from these pansies GO SURFING! GO BIKING! YOU ARE LOSING PRECIOUS SECONDS, MORTAL! USE THEM ALL UP! Seriously though, if you are really stressed out then sure lying down for a half hour is a great way to unwind and kill time, but it often won't take your mind off of something IMO. You need some activity, even simple and banal, to help occupy your mind and let you chip away at your problems subconciously. I used to assemble survival kits and stuff in my teens to help me take my mind off of things. Go camping, learn survival skills, stuff like that. If on the other hand you aren't stressed and just want a new hobby then why not take up surfing or skateboarding or piano or paintballing? You can teach yourself all this type of stuff with minimum cost.