Parks. We are pretty lame and don't do much when we have weekends. I just spent my morning digging clay and rock at church and got home and my kids and husband are still asleep, so I've been working a few hours away from home, and now have a lot of work to do at home. Our house is pretty well neglected inside just because when the weather is nice we at least get out to the park or play outside.
We are fortunate to have some good fishing areas out here and my husband and I take the boys. I usually walk the trails with one and he fishes with the other since I have not bothered to get a license yet. That is free and gets us outside and the kids love it. I also try to always take a bucket or critter keeper and find toads, snakes, minnows, or crawfish with the kids. We went out and caught fireflies the other night. Sometimes just those isolated fun moments make a huge difference. We are going to try to get up to my in-laws' camp a few times this summer. They just bought one and are fixing it up, but it is over 3 hours away and our gas budget is also pretty tight. You could do a lot outdoors if you have a sling or backpack carrier for your 6 month old. I had a book when I was little called The Young Naturalist and it had great activities for kids with nature, including creating an eco-system in an aquarium, different projects like building a bat house, creating a pond, studying animal tracks and leaves. Activities like that would be great for your older children. There is also a website called lostladybug.org which is a project for kids to do taking pictures and reporting on ladybugs. That might also give you some focus for your time outside.