I have three hours (let's consider this a morning, afternoon, or evening)
1. You'll have to do a little work to see if this is a possibility for you, but three hours is a good chunk of time to donate to your local food bank. Click here to see what's close to you and what current volunteer needs exist. Feeding America has a network of over 200 food banks serving all 50 states, the District of Columbia, and Puerto Rico.
2. If you live in Chicago, DC, MPLS/St Paul, New York, Orlando, San Francisco or Seattle you have a great way to volunteer and have fun at the same time. One Brick brings volunteers together to support other non-profit organizations. You commit to an event, so it works well for those who are too busy to volunteer on a regular schedule. It's a trickle down kind of organization since everyone, including the staff, are volunteers. In 2009, volunteers donated a total of 65,000 hours to the communities One Brick serves. Updated 6/28/14: They are growing and have more locations now!
3. I'm very excited about this one because EVERYBODY can do it, and you'll benefit as well. Guess what... I'm going to send you to clean out your closet. Or your basement or your attic, or your linen closet.... possibly your garage. What you are doing is looking for items that you don't need anymore but are something someone else could use (i.e. not junk). That's Step 1. Step 2 is to bundle these things up and take them to the nearest charity thrift shop. How easy is that? If you need help finding a local charity shop, visit this national directory of charity-driven thrift shops. Dare I say that Step 3 could be doing some shopping there as well?