Ha Long Bay.
Dont let the guides or brochures fool you into believing its only a 2 hour journey from Hanoi, it is actually a 4 hour drive each way.
I let the owner of the hotel i was staying in Hanoi talk me into a 2 day/1 night stay in Ha Long Bay, and after this trip, i was really glad that i allowed him to do this. I ended up booking with Legacy cruises, with a cost of $130 per person, but seeing as i was travelling alone, i had to pay an extra $30 single supp charge. Now, it might seem expensive, but honestly, this had to of been the best £100 i have ever spent on any holiday anywhere