I highly recommend Viator for 'add on' side adventures. We did a couple tours with them in italy (Angels and Demons was AMAZING) and they are really great. We already booked the light show on the pyramids, Alexandria, Memphis and the Hanging Church (and DH is trying to figure out when we can do the Nile Dinner Cruise without being zombies). No... I don't have a vested interest in Viator, I only have the great experiences from the last two weeks to go by.
They aren't Disney by any stretch of the imagination, but they are very knowledgeable, they limit the number of people on any given tour so it isn't unwieldy, and they do a great job of explaining and engaging you during the trip.