-Take up yoga. It feels amazing!
-Go to music shows/art shows etc...a lot. You'll start to befriend people who you see there often.
-Go night-wandering... which can sometimes include:
-Create found art. (Make things out of trash and leave them for others to find) which goes along with...
-Ne experiments. Ne experiments would be something like doing something small (subtly changing the environment where a lot of people are present, leave a note a weird/cryptic note on a public seat, leaving little mementos or nick-nacks in random/unexpected places... etc) that has a chain-like reaction of creating thoughts in others or in some cases actions.
Illegal (don't read if you've got a problem with that!):
-Graffiti thought-provoking phrases in public places or
wheatpaste interesting posters in public places; they're nearly impossible to take down: Wheatpasting: How to - Worldwide : citynoise.org
Also: hear are some interesting posters to paste :CWC Tools : Posters
-If you live near train tracks walk down them. If you happen by a freight train with an open car get on it for a while. You could also graffiti the train... think about how far it is likely to travel and how many people will see it.
-Steal things. See how much you can get away with by learning how to read people.
-If you do drugs, get really high/trip and go to a museum (especially art museum).
-Explore abandoned places/urban decay...you would be amazed at what you find.
I am not saying I do all of these things or endorse them. I've collected them from activities I partake in and some of my friends partake in. Just sayin' is all...