Most hotel in Hanoi will allowed you to keep your bags there, hoping that when you return, you will stay there. When we took our train tickets from Thanh Vinh Tourist Hotel, they also offered us to keep our bags there, plus free usage of bathroom. However, I would like to try out other hotel, and also the fact that I was a bit disappointed with the Halong Bay tour package that they gave us, we rejected their offer instead.
Then we found Wing Hotel and Cafe, under Minh Toan Hotel and Tourism Co. We booked with them a night stay after returning from Sapa for USD $8, bargained down from USD $10. They allowed us to keep the bag and usage of bathroom for free too. The receptionist there was a very cheerful and friendly girl, even promised us we can always knock the door if the train back from Lao Cai reaches Hanoi too early. What she didn’t tell us was there is extra charge for early check-in.