We've been to Ko Phi Phi and Saigon. You only have one day? To get to Phi Phi you will have to take a taxi to the ferry, and take a couple hour (If I remember) ferry ride to Phi Phi. That is a lot of travel time both ways. We stayed at the Holiday Inn on Phi Phi and really liked it. It is nice and quiet (you have to take a water taxi to get there) and our bungalow was fine. I found Phuket to be too crowded for my taste. We stayed at some resort out of town (excellent).

Saigon is one of the few places I was very afraid to cross the street. Think of a crowded street in Bangkok and add a lot more vehicles. We enjoyed our visit and yes, we went on a tour to the Cu Chi tunnels, which we thought were pretty amazing, especially since my father in-law fought for the US army in Da Nang during the tet offensive. If you do go to Saigon, bring very little stuff as there are plenty of bargains to be had.