Halong Bay is probably a must visit in you're in northern Vietnam. So being a first time visit I did what all tourist did, even though I hate doing that. I took a cruise out to Halong Bay, boarded a smaller boat, toured around the limestone structures, watched how the locals live on their boats and finally went to Dong Thien Cung to look at the stalactites and stalagmites. All very touristy. Here's a photo I took when I was inside Dong Thien Cung, and I have to admit, it was quite remarkable inside.