Nong Khai sits on the banks of the Mekong river. There is a border crossing to Laos here, and the Thai-Laos friendship bridge.
It is a relatively quiet town, with a few tourists passing through from Laos. On a Saturday evening there is a large market along the waterfront. It is popular with locals and has a good range of street food stalls. You can also see good sunsets across the river.
Across the town is Sala Keoku, which is a sculpture park. It was started in 1978 from the ideas of Pu Bunleua Sulilat. There are lots of fantastic buddhist/hindu concrete sculptures, with the largest around 25 meters tall.
If you head 80km west along the highway from the town across Nong Khai province you can visit Wat Pha Tuk Suea. The temple is situated up in the hills. Now when you visit you can see the small skywalk they have built. This skywalk sits out over the cliff edge, and gives massive views down to the province below, the Mekong river and Laos.