It´s a nice sunny day, perfect to be outside. But you have to go to the gym for your weekly exercise so being in the sun will have to wait. Hang on, what was that? You have to go to a closed gym to workout and will leave the benefits of the fresh air for another day?

Who said you need to go to the gym to get a full workout? Bring your gym outside with you instead! If it's a sunny day, why not go for a run in the park instead of running on a treadmill in a sweaty gym? Unless you live in a place where pollution is a big issue, NY for example or larger cities you should be able to find a place to exercise outside and be able to enjoy to benefits of doing so.

You can do push-ups and sit-ups in the park, use tree branches to lift yourself up instead of lifting weights. Bring some active kids and run after them and lift them up in the air. Free weigh training that the kids will love. Bring your dog to an agility park, great exercise for both of you.
Suggest to your yoga teacher that your have an outside class if the weather permits. Or bring your own yoga mat outside.
Go out on the country side and help on a friend's farm. You will get more exercise than you can imagine.
Call all of your friends and ask them to come around and play night football, you'll sleep like a baby afterwards.

You see, you can still enjoy being outside and get a full workout without going to the gym. Maybe replace one of your weekly gym sessions for an outside session?