Just go ahead outside without them. Go for a walk, dig some holes, or whatever. Just go.
If you want to find a few more active friends then go where active people hang out, such as the gym, a bicycle shop, the pool, a sailing or canoeing or hiking club, or the nearest park.
When you go out take a frisbee or a ball with you and ask a likely looking person if they would like to play catch. Even better, take a dog if you have one – that’s a great way to meet people who like to go walking.
Volunteer somewhere that has to do with being outside. Help coach a kids’ team, offer some time at a nature center, hang out with bird watchers, participate in Earth Day or Arbor Day activities, adopt a section of road and invite a friend or two to help you pick up the trash, or any other number of outdoor activities.
You don’t have to give up your x-Box friends, just add a few outside friends to the mix.