Nong Khai does not have an airport, but it's only about an hour away from Udon Thani airport. Nong Khai is also the end of the Northeastern Line of the Thai state railway. There is a train service across the Friendship Bridge into Laos, but the service only runs from the Laotian side of the bridge to Nong Khai. Coming from or going to Bangkok requires a change of trains in Nong Khai to reach Laos.
Bus is another option. There are regular buses from Bangkok's Mor Chit bus station, as well as nearby provinces such as Udon Thani and Khon Kaen.