The Mill Avenue District is the ONLY place in Arizona where you can actually walk from place to place. You can park your car and spend all day there! It is also the only downtown area that is waterfront, so you can relax while walking along Tempe Town Lake, or even rent a paddle boat from Rio Lago.
There a several great restaurants in the Mill Avenue District, such as Caffe Boa, House of Tricks, and La Bocca Urban Pizzeria & Wine Bar. There are also several major events that are definetly worth going to, like the twice annual Tempe Festival of the Arts!
Downtown Tempe encompasses the Mill Avenue District (Mill Ave), the Lake District (Tempe Beach Park), University District (College Ave), and the Farmer Arts District (Ash Ave). There is LOTS to do.