Hi, Angela. I can strongly recommend AmeriCorps - I spent a year building houses with Habitat for Humanity. I think the satisfying, outdoor, physical labor is particularly appealing to those of us who like to be outside. I liked it so much I took on a staff roll with Habitat for two years following my service. I also know there are various AmeriCorps programs that deal in conservation.
Internationally, I can recommend Little Children of the World. I spent five months with the organization helping with their health, education, and livelihood programs at their primary site in Dumaguete, Philippines in 2001. Aside from their mission, what I found so appealing about the organization is how straightforward they are about costs (so many international volunteering programs have high application and other fees). With LCW, you cover your own airfare, and then pay a modest weekly amount that covered housing and lunch five days a week when I was there. As I'm typing I'm reminiscing about my time there: truly amazing. While there, I had a number of friends that were doing Peace Corps, but I found this a nice alternative since I didn't have two years to devote at the time (I was in college).
If you PM me, I can give you all kinds of details about either of these options. More generally, I highly recommend following through on this plan!