What sort of challenge would YOU like ? Because when we exercise, we should do something we enjoy. that's because when we enjoy doing something, we look forward to doing that something each and every day. What is something you've never tried and would really like to do ?
Would you like to rock climb ? If so, check your area of indoor rock climbing gyms. Have you ever considered taking dance lessons ? How about signing up for some dance lessons. take your pick, salsa, tap, ballet, etc...
How many push ups can you do ? How about trying the 100 push up challenge ? If you want to be stronger, learn to do military style push ups. Go here for more info. hundredpushups.com/
Ever consider running a 5k ? If not, sign up for a 5K in the spring and start training with a good Couch to 5K program.
Exercise can be anything you want it to be. Therefore it's important to find something you really enjoy because what happens when you get bored ? We can only motivate you for so long. After a while, you're going to have to start motivating yourself.