July August is the rainy season in the north of Vietnam, however, it's just a quick heavy rain for half an hour in the afternoon or evening to cool down the weather only and it will not affect to your travel. Sapa is a nice town but quite touristic. Ha Long Bay is very nice and you can do just a day trip or stay overnight on a cruise/junk.
If you would like to have the deep impress of the authentic of Vietnam, I can help you making a tentative itinerary to the off the beaten track. If I can be any further of your assistance, please do feel free to contact me. Im Phuong, my skype is chuckeechuckee, Im online from 9:00am to 7:00pm (GMT+8) or you can contact me via
sales@customvietnamtravel.com. Im more than willing to assist.