Welcome back to the age of steam! grin I'd suggest a campfire or camp stove will generate enough steam to power all sorts of "pocket" engines and generators. Some of the hobby steam turbine generators I've seen weight only 3-4 ounces. YouTube has hundreds of ideas for this kind of thing and might be fun in the field.

http://www.youtube.com/watch?v=zD2ERF2s8hE
http://www.ministeam.com/acatalog/Steam_Engine_Accessories.html
http://www.youtube.com/watch?v=ENsnutreEjQ
http://www.youtube.com/watch?v=ZPX2grfJwsg
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paul, texas KD5IVP