some of the highlights from my last trip...
: nice little beach town, short air hop or train ride east of Saigon, scuba diving, fishing, good seafood, nicer hotels w/air, excellent hot springs / mud baths, tourism, motorcycle rentals, etc
tunnels: northern district of Saigon, huge network of complex tunnels used by viet cong back in the day, shooting range where you can handle/shoot many of the weapons used during the war (full-auto, semi-auto)
(cuu long): not too far west of Saigon, take a boat tour, very different world from the city, locals making their livelhood from boat (aquaculture), long history, big time rice and fish producing area, very cool...
market: old school shopping area in Saigon's district 1, get lost while shopping, lots of action but practice saying no and don't get pulled into a corner, hire a shopping helper (cheap) that speaks english