What a great location! Tempe has given us a great gift in Tempe Town Lake, with options for many! While as many point out, it's not much of a lake (there is no water-skiing!), it is such a better landmark than a flood/drainage area that is dry the majority of the time!
There is a path that surrounds the lake which is great for walking, and it also gives great biking access in the area. I always enjoy watching the boaters and there are numerous locations for a nice relaxing picnic lunch (a much better way to spend an hour in the middle of the day!).