I have been entertainment chair for our local Relay for the past three years.  I have found that live entertainment has worked well.  I have booked from kids gospel group to a full live band.  Last year I even booked some guys that impersonated the "Soggy Bottom Boys" and they were a hit (and will be back this year).  I try to book the band last as people will stay to hear the band.  There are lots of local talent (be sure they have talent) around to be found.  People who sing at church or at fairs or bands that play locally.  When you tell them it's for your Relay, they will gladly volunteer their time.  For the individuals this is a chance to show off their talent.  If you get the word out that you need performers, you'll be amazed at how your phone starts to ring.

It's better if you have a flatbed trailer (and steps) for them to perform on.  You will need some sound equipment and usually the owner of the equipment will stay and operate it becasue it is so expensive they don't loan it out.  Tell them what it's for.  You will be amazed at the people that will be willing to volunteer their time.