If you're in Saigon on a Sunday night, then beg, borrow, or rent a two-wheeled vehicle and join the throngs for di choi. It's basically a party on wheels, where everyone just rides through the downtown streets until the wee hours.
If the heat starts to get you down, there are several water parks where you can splash around to cool off. Close to the city centre in District 11 is Dam Sen Water Park; just north of the city in the Thu Duc District is Saigon Water Park; others are Water World in District 9, Ocean Water Park in District 7, and Dai The Gioi Water Park in District 5.
Galaxy Cinema at 116, Nguyen Du, District 1, is also a favourite among locals and bored tourists. They show up to date movies in a 4 screened cinema.
Visiting hair salons is also a must do for tourists, as Vietnamese are famous for it. Hair wash, manicure and pedicure cost no more than USD10. One nice place to go to is Vu Salon @ 210, Tran Quang Khai, District 1, within city limits.
BUY
Ben Thanh Market, west end of Le Loi. Probably the largest, offering a wide spread of, well, pretty much everything from Ho Chi Minh T-shirts to cosmetics, diapers, pickled plums and live ducks. Popular with tourists, so prepare to haggle (your luck may vary with this, as 'fixed' prices are being introduced).
Night market (just outside of Ben Thanh market). Here you can enjoy many kinds of different food and drink, and go round to do your shopping as well. Open from 17:00 (when the Ben Thanh market closes).
Saigon Square, corner of Hai Ba Trung and Nguyen Du. Some of the former Russian Market stores have moved here. Very popular for hip, young people; you can find cheap watches, DVDs, T-shirts, jeans, shorts, slippers, etc, and there's a nice supper market in the middle of the square. Open 09:00-19:00.
Tax Department Store, corner of Le Loi and Nguyen Hue. Formerly the Russian Market, this is now a rather sterile department store of sorts filled with stalls selling touristy kitsch, although the selections get better as you ascend the levels. There's a good supermarket on level 2. If you are traveling here by taxi, the new name may be met by blank expressions from taxi drivers. The old name seems to work.