The Move On website has action updates that will give you some ideas: http://www.moveon.org/
Also, call up your local Democratic party. Find out who your precinct chair is and give him/her a call to let them know you want to help.
You could volunteer to work for a candidate. Here are some of the things you could volunteer to do:
1. Distribute door hangers
2. Stuff envelopes
3. Phone banking
4. Register people to vote
5. Distribute lawn signs
6. Wear a t-shirt, put up a lawn sign, put a bumper sticker on your car expressing your political viewpoint (be ready to discuss this with people who agree or disagree with you).
7. Sit at at table at an event and pass out information, bumperstickers, buttons and so on
8. Write letters to the editor
9. Go to public meetings. Ask questions.
10. Honk & Wave (that's when a candidate stands by a busy street with a crowd of supporters)
11. Talk with or e-mail your friends and neighbors about your political opinions (this is how the Howard Dean campaign is generating such strong grassroots support).
12. Look on the Howard Dean website and find a local Dean meeting to go to. http://www.deanforamerica.com/
Even if you aren't sure your're for Dean, they are generating some geat activist ideas.