Years ago I took a buyout offer at work that had a higher percentage of salary payable in a lump-sum severance (75% of a year's, vs. 40% thru their more conventional offers). The money was payable 50% upfront, 50% in 7 months with documentation. They would also continue benefits for 3 years. BUT, as one of the strings attached, you had to either teach, become a student, or do volunteer work for a certain number of hours in those 3 years and document it every year.

My goal was to use the buyout proceeds for a year of travel around the world (a lifelong dream, as I hadn't been able to do it at a younger age). So I looked into the sort of thing the OP is mentioning. The company threw cold water on a lot of ideas when they said the volunteering couldn't be outside the U.S. (Truth to tell, I don't think the company liked what I was doing much or imagined that that buyout offer would be used in such a way).

Anyway, because of tight finances and a general frugal nature, I didn't like most of what I saw in the "volunteer vacations" research I did. I was OK paying the transit costs to and from the sites, but the daily costs often were as high as vacation costs (for me, at least), and that didn't seem right.

What saved the day was Passport in Time, a little-publicized U.S. Forest Service program where you go to national forests to help the archaeological people ("arkies") on projects. The projects are provided for by the National Historic Preservation Act (signed under Nixon). Some places have prehistoric (Native American) finds and others have historic ones.

What really helped in my case was that there were a few projects listed in their bi-annual guide where you could be lodged at no cost in quarters used for firefighters during fire season. I made sure to go for those. Sometimes it wasn't a slamdunk, as the program had a dedicated corps of retirees who'd do multiple projects, and some projects had too many volunteers.

Anyway, I'd either get a low airfare deal or use miles, and rent by the 2 weeks on cars, and would have to pay for my own food (though the quarters had refrigerators and kitchens). Did 2 projects at the Plumas National Forest in gold rush country, sorting and bagging historic artifacts. Did a summertime dig in southern Washington looking for arrowheads, and a cabin restoration of a Depression-era log cabin in southern Oregon where there turned out to be tension between a hired masonry contractor and the project leader. Also a dig in far southwest Kansas where sun was a real problem, and where the whole town showed up to see what was going on. Digs were hard work, as you'd have to level off your "unit" every 10 cm and have lots of attention to detail. I usually had to sift, and wasn't that good at telling a "cultural" artifact from a rock.

But it was great experience. I would do a 2-week trip every 6 months for that (to get in enough hours) and work that around my other trips.

More recently, a sister-in-law and niece did a volunteer/mission trip through a megachurch group to the Dominican Republic that had such a high "co-pay" that I had to bite my tongue to avoid saying that you could do a resort trip for that if you shopped it carefully!

I think there's also a deal where you can teach English in Spain for 4-6 weeks and get room and board for free (You just do airfare). Transitions Abroad magazine has lots of listings for things outside the U.S.