Marcie,
I would definitely recommend a guide for the Falls. We were met at the airport, taken to the hotel (Sheraton), checked in by the guide and then started a part of the tour. Since you will only be there five hours, he/she will see that you make the most of your time. There are different paths to take to different sites and your guide will be sure you get to them with ease i.e. Devil's throat, the train ride, etc. Hopefully you will be able to go on raft/boat under the Falls. It is so much fun. Be prepared to get wet either from under the Falls or from the spray at Devil's Thoat. I took a zip lock bag - large to keep things I didn't want to get wet. Magellans as a great waterproof bag that is also good. You will love this trip.
Deanna