Now that you have an empty nest, life may seem a little quieter than it used to be. There is no longer a parade of your kids and their friends tromping through the house, asking for food and needing to be cleaned up after. Is this really something to be sad over? You've done your job as a parent: you've raised your children right and now they're off to college or work. Why not reward yourself?
First, think about the space that isn't being occupied anymore. Oh, the possibilities! If you've always wanted a gym, now's your time to invest in a treadmill and a weight set. Perhaps a home office has been a pipe dream of yours. Well, it doesn't have to be anymore. That antique roll-top desk would look awfully nice once you clear out the posters and debris left behind by your son or daughter. Or maybe you just want a fancy guest room to host friends and family members. Make it your own special project that will provide you with the space you previously thought was just too pie-in-the-sky.
Don't feel guilty about using the space for your own personal wants or needs. This is your house, and you've spent the last 18 or more years catering to your son or daughter. If it helps you feel better about them going off on their own, then it's what you should do. You can always leave a bed in the room for when they come home to visit.
Now is also a good time to see the rest of the world. Your travel expenses are much lower, considering that you won't need to tote around the kids. Additionally, you and your partner don't need to take their desires into account. Never got to backpack through Europe in college? Now's your chance! Was scuba diving in the Great Barrier Reef too expensive in years past? Look for travel deals and take a trip with your partner or a friend.
Also, a great way to occupy free time and maybe meet new people is to take a class. Whether you choose to learn a new form of exercise or a new language, you'll feel better using your time to improve yourself.