Although you said that working with children wasnt your thing I worked at a school and orphanage in Tanzania for 3 months last year and was surprised with how I coped. The best way to find cheaper volunteer opportunities is to apply directly to the charity. I worked at Peace Matunda School and Orphanage - www.peacematunda.org. They were very helpful throughout my time and always willing to help with enhancing my time. Good Luck with whatever you choose to do. Volunteering is such an amazing experience - one I would love to repeat!

I spent a couple months volunteering with pisco sin fronteras (www.piscosinfronteras.org) which is an organization in Peru doing construction work in response to a 2007 earthquake. HODR was there initially, but eventually left, so this organization is doing more long term rebuilding. Mostly what they are doing is building very basic homes for families, usually out of brick and concrete although they're starting to get more into wood now. There isn't a fee to volunteer with them, but if you stay there and eat your meals there it's around $5/day. They really like longer term volunteers with construction skills, so it seems like you'd be a good fit. I had a great experience with them. I'd give a lot more details but I'm not sure where to start and they've got a pretty good informative website so you should start there if it sounds interesting, but please email me if you have questions, I'd love to try to answer them :)
Also, I found the website volunteersouthamerica.org really helpful when I was trying to find a place to volunteer, it's a huge list of no cost and low cost volunteer opportunities (all in central or south america, though.) There are a few other construction type opportunities, but they're mostly focused on kids.

Thanks for the info everyone. I see that there is a lot of need for teaching and work with children, but I am not sure how great I would be at that. geegollygosh, I do have some questions for you. What were common ages of the volunteers at Pisco SF? Can you describe the atmosphere, typical work day?

The average age was relatively young-- most people 18-28ish, but with quite a few people in their 30s, 40s, 50s. It was sort of a backpackery atmosphere- the social atmosphere really depended on the group that was there-- when I first got there, people were going out to bars all the time and were more wild, but by the time I left it was a very laid back atmosphere with people mostly sitting around the fire talking. Overall, though, it was always a great group of enthusiastic and interesting people and (almost) everyone was there to work. There were anywhere between 50-90 people there while I was there. The administration is pretty laid back, it's definitely not a top heavy organization, for better or worse (usually better).
typical work day-- You could choose what project you wanted to work on each morning from a variety of projects, some people stuck with the same project each day, other people jumped around. The work was basically digging trenches for the foundation, then filling them with cement, laying brick for the walls, pouring a cement floor, and putting on a bamboo roof. Nothing fancy, but much better than what a lot of people have now. Everyday there was a morning meeting at 8:30, then you went to site from 9- 4 or 5. workdays were mon-fri and a half day on saturday, but if you really wanted to work on a sunday, there was usually a project you could help with. Sometimes it was frustrating because we didn't have the materials we needed, or the guy who was overseeing the project (that the family had hired/appointed) didn't show up, or the cement mixer broke and we'd have to mix the cement by hand, etc... But sometimes it was really great, too, and we'd get a lot done and have a nice conversation with the family and go home happy. It really varied. There were other projects besides construction also-- they had a biodeisel project, and people would help out at a local school, and sometimes there were other things going on too.

I just came across this site which rates and reviews volunteer places, although its a bit sparse at the moment
[http://sites.google.com/site/volunteerreview/]
It may be of use to some of you