By daily basis I assume you mean just pay for the day? Rather than every day? One of the best gyms in Hanoi - as far as I've heard - is NShape Fitness. It's in Ba Dinh so will cost about 40,000VND in a taxi each way from Hoan Kiem. Their membership fee is really high but they do a daily membership of about $15 I think. www.nshapefitness.vn
There's no pool there but the Army Hotel has a pool you can use for $5.
I'm not sure about any others - I looked into it with no joy though apparently there are lots of local gyms about.
I agree with abc about the local gyms being basic. i live in thu duc district of HCMC (about 15km east of district 1) and while we do have 10 or so gyms near me, they all are very bare-bones (mine has 3 treadmills, 2 stair steppers, 2 stationary bikes, a few weight lifting machines and various free-weights). However, the cost is very cheap- around 120,000vnd per month!! If you want to work out only for a few days while in Saigon, maybe you can join a local gym for a month and only use it as long as you are here... It may be hard to find them though- I had to have my Vietnamese friends show me where the gyms are as I can't read Vietnamese
There is an excellent gym called the Lan Anh Country Club about 5km from District 1, its has about 5 tennis courts, a outdoor pool, a restaurant and micro-brewery, and a pretty big gym. There must be 20 running machines, cross-trainers and bikes. It also has a big free weights area and a lot of weight machines and benches etc. Its definitely worth a look, plus its a mixed gym unlike most in Saigon, or so i have read. The address is 291 Cach Mang Thang Tam Street, Q10. Entry for the day for the whole complex is only 70,000 VND about 3.50 Dollars. Oh yeah there is also a big cinema room just above the gym area. You pay on arrival at the main gate, you can take a look around first as i did, and you get a towel and locker key at the gym. Travelling from District 1 to the gym costs around 30,000 VND on a moto-bike. Thats all the info i can think of, hope this helps....