If you're like most everyone with a 9-5 job, chances are that you spend a good majority of the day breathing the stale air of your office. On the weekend, take advantage of not having to be in the office by forcing yourself to stay outdoors for the entire day. Recruit your husband, boyfriend or best friend to join you so you don't get bored (or cheat).
Aside from the fresh air and change of scenery (literally), the best thing that comes out of planning a day outside is that outside activities typically burn a lot more calories than the sedentary indoor lifestyle.
So take a walk. Go for a hike or bike ride. Walk your dog. Play tennis or golf. Wash your car. Visit that outdoor mall or nearby town you've been meaning to peruse. Garden. Go to the zoo or the botanical gardens. Go to the beach or pool and actually swim. Better yet, learn how to surf. Pack a healthy picnic. Even if you and your partner in crime opt to just sit at a park and read books, you both still have to get off the couch to get there. Be sure to bring along a Frisbee in case you feel inspired by others getting moving at the park.
My husband and I actually do this every so often. We always find a ton of things to occupy our time and we always feel like we've had a really full day. I promise you'll be amazed at how tired you'll feel... Sure they say that all the fresh air makes you tired, but I'm willing to bet it's also because you end up moving a lot more when you force yourself to stay outside for five or more hours.