The hike/walk up A Mountain is immensely easy and anyone can do it - even kids, old ladies, and those of us who are totally out of shape. Look for petroglyphs on the rocks and enjoy views of Tempe, Downtown Phoenix, and the valley from the top.
If you're on Mill Ave., visiting ASU or Sun Deveil Stadium anyway you should totally go for it, but I wouldn't go out of my way to do so.