There is a park by my house that has a trail around the outside with stations set up for performing various strength training exercises like pullups. I got into the habit of running around the trail a few times and then stopping at each one of the strength stations.
My general fitness improved almost immediately. I love doing it this way because it is so cheap and convenient. I don't have to pay for a gym membership or any special equipment. I just throw on my running shoes and head for the park. I have noticed these kinds of setups in a lot of parks. Anyone looking to improve their general fitness should head down to their local park and see what it has to offer.