My top 3;
1) Talk to 10 people a day (yes 10 not 3 you wuss
). Strike up a conversation and figure out there dreams, hopes and ambitions.
Listening to people telling there story without mentally judging them is a very positive thing to do, it can transform some people's life and who knows perhaps you pick up new friends through this.
2) Vow of silence. Would be interesting to see the results of this.
3) Complete reversal of your lifestyle. Would be interesting to see if you'll have trouble going back to your current lifestyle again and how much your 'new' habits stick.
The number 1 is way ahead of the others though. You can post a short synopsis (without mentioning names) from each conversation on the blog.
If you have any reservation about talking to strangers then by the end this should be completely gone.