It's an easy hike, but extremely scenic. Lots of wildlife, including bats if you go during the right time of year. Great for bikes, dogs, and even runners (although it does get blazing hot come summer). You can just walk through the tunnels, or take it further and head all the way to Hoover Dam.
If you want to make it more challenging, I suggest running or jogging the entire length. I think round trip all the way to the Dam is 6 or 7 miles, and there are some stairs in there, too.