@Ryan *****
go to Laos, Nong Khai to Vientiane by the friendship bridge
You can fly to Udonthani from Bangkok however many people go either by bus or by train from Bangkok to Nong Khai then tuk tuk to the friendship bridge to cross over
You can catch the night train with sleeping- very nice ride approximately $1,000 baht
They have upper and lower bunks, lower is bit more expensive but worth it, they have power points next to each bunk for charging phone ect
Train arrives in Nong Khai in the morning, you can cross over and straight back in about 1 hour depending on how many ppl are crossing then night train back to bkk, train also goes through Korat Nakhon Ratchasima train station