My 3.5 hour flight on Thai Airlines left Colombo at 01:30 and arrived in Bangkok at 06:30 with a 2 hour time loss. Everything was smooth and there weren't any long lines.
When I checked in for the flight they asked for an onward ticket. The guy was carefully examining my onward ticket so I forgot to ask for an aisle seat. He gave me a middle seat but the plane was only about half full so I moved to a row with an empty seat next to me.
In Bangkok, I killed a little time at the airport in a coffee shop since my hostel room wouldn't be ready until the afternoon. I took the airport train to the MRT and then got off at Silom and walked over to the hostel and left my big bag there.
Then I took the MRT to the Chao Phraya river to ride the river boats and see the sights. There were tons of tourists filling up the river boats. I don't remember the river boats being so popular when I was in BKK five years ago but maybe it was the time of year or the time of day.
Finally got my room at around 14:00 and slept a few hours. Then went out in the evening first to Lumphini Park and then messed around in Silom and Phat Pong. The hostel is at a great location right in the middle of everything on Silom Road.