While Phoenix has enough going on to amuse you for months on end, you should definitely take a morning or afternoon to check out the neighboring city of Tempe.
Minutes away via light rail and home to Arizona State University, Tempe is the quintessential college town and has a lot to offer in a small radius.
Whether you bike it or rail it, make your first and last stop Mill Avenue. The main drag through the center of town, Mill Ave is home to the majority of Tempe's shops, bars, and restaurants. Take some time to browse through the selection of herbs at Happy Healthy Horney High and cure whatever ails you, grab a taco and a "Shocker" at Fuzzy's, and sun yourself on the patio with a craft beer in one hand and, heck, a craft beer in the other at The Handlebar.
After you've satisfied your many hungers, head over to the Town Lake for some catch-and-release fishing, sand beach volleyball, and a little watersports (of the boating variety, pervert). From paddle boating to kayaking to motor boating, Tempe Town Lake Rentals will hook you up with a sweet, seaworthy vessel at a reasonable price - lessons not included - so handle your craft with care, captain.
While technically on the Phoenix side of Town Lake, Papago Park offers some beautiful desert scenery and some light hiking opportunities a few minutes from the Tempe town center, including the famous Hole in the Rock, which is exactly what it sounds like.
From the park, you have easy access to both the Phoenix Zoo and the Desert Botanical Garden, though if you'd like to save some money, you can get a decent view of both from the overlook that serves as the base of Hunt's Tomb (but let's just keep that our little secret). George Wiley Paul Hunt was the first governor of Arizona and has a pretty cool origin story and was entombed in an even cooler memorial.
The cool thing about Tempe is that everything is in really easy walking distance and can be covered in less than a day, making it a great, and accessible, detour from Phoenix.