The rice harvest season (Mid August to early September) in Sapa is over at that time but it is still worth a visit.
After Halong Bay, you can consider doing Ninh Binh.
If floating market(s), head to Can Tho.
Currently, it is just a single entry visa (Evisa) so you need two separate evisas to enter and re enter VN (One can hold more than two evisas).
Use a first evisa to enter VN, fly to Cambodia for 3 days and then re enter VN under a second evisa.
As for visiting the floating market in Mekong Delta, Can Tho is a good place for that. Also, there are two floating markets in Can Tho, one is Cai Rang (Big trading boats, wholesalers) and the other is Phong Dien (Small trading boats, retailers).