I know I hate it when someone answers a question with a question, however, what do you like to do?
Spend a day in downtown Phoenix and visit the Phoenix Art Museum (PAM), Heard Museum, Library, Science Center and Children's Museum,or pick a few that interest you. There's free parking at PAM, the Heard and Library. The Heard is the farthest north. Park at the Heard, buy a day pass for the light rail, explore the museums & library and explore downtown Phx. Have lunch at Friday's Front Door. It's part of Chase Field and the tables overlook the ballpark.
The Desert Botanical Gardens are a must do. Old town Scottsdale is fun to walk around. If you like the college scene, visit Mill Avenue and ASU. You can get there from the light rail. We have tons of shopping from high end to discount, excellent restaurants, golf, tennis, hiking,spas, casinos, and a zoo. I'm not into zoos, however, ours is large and spacious.
There's a bunch of restaurants at Westgate located near the football stadium. If you're in town on game day, I don't recommend going to that area unless you are going to the game.
Unless you're going to visit my mother, plan to commute.