What about getting some of the Boy Scouts merit badge books that look interesting (you can get them at any Boy Scouts Store for pretty cheap) and do some of the activities in those? You could gravitate to the more active badges or the more sciency badges. There are literally more than 100 to choose from and they might give you (and/or your sons) some structure to work from and some ideas you hadn't thought about, but with with no requirement to do anything you didn't feel like.
Off the top of my head I'm thinking archery, canoeing or boating, geocaching, cooking (but outdoors on a campfire they build themselves), hiking, beachcombing, swimming, camping, rockets, whittling, ropemaking and knot-tying, etc. Or what if they work to build a treehouse or a wooden toolbox or just some random wooden creation using scraps of wood and a hammer and nails? Or change the oil and wiper fluid in your car?
