Just done a visa run to Kanchanaburi, so I thought I'd share my experience in case it helps any other newbies to Bangkok.
Met agents at On Nut KFC at 4am. Once everyone was there, we set of at 4.15am. It was a 9 seater VIP minibus with reclining seats.
The journey took 3 hours, with a 10 min stop for refreshments along the way.
Stamped out of Thailand at the border and gave passports to the agent. They filled out TM6 forms and took photocopies of passport page for Myanmar Immigration. Signed TM6 and photocopy and proceeded to pickup which took us all through the border and into Myanmar, Waited by some little shops for about 15 mins while the agent sorted out the passports and then back to the border complete with Myanmar entry and exit stamps. Queued for about 20 mins at Thai Immigration and then stamped back in for another 30 days.
Back into the minivan at 9.30am and back at On Nut at 1pm.
Cost = 1900 baht
Agent =