I'm finding that initially, it's expensive to do. Like, we went out and bought tennis rackets for all of us. We are starting to bike ride too. We walk a lot. Gardening!!!! (Girl, I know you have a huge property!) Swimming, etc. Basically, whatever you can do outdoors, do it!
I know your girls are still young (3 and 7?) So some things will be harder, but there are ways to work it. Like, for me, I am wanting to ride my bike more instead of driving. I live in a community that makes that easier. For kids, there are trail a bikes and trailers - you can often find those on craigslist for lower prices. WHEN I get my bike, I plan to ride to the library, the grocery store and the farmer's market.
You can always rent canoes or kayaks at bigger parks. There are tons of state trails to walk all over the country. Once you get started, you see there is just so many ways to get more active.