It’s not really practical to do in a day trip from HCMC as the best floating markets wind up before you will get there. You can still have a good day doing a tour though, but I’m not sure you will get much cycling in. You can book a tour at Sinh Cafe in HCMC – it will be cheap but you will be with a bunch of other tourists and will be herded around to the usual places – I still think it’s good though. Otherwise there are some private operators that are more expensive but will give you a better tour and you are more likely to get your cycling. Search TA for private tour guides.
In my opinion the floating markets are not worth the effort required – when we went there were probably more tour boats than local boats. The Mekong is a great experience with or without the floating markets.