Hi Ian,
If by "tubing", you mean the activity described on Wikipeida, I haven't heard of it in Thailand: http://en.wikipedia.org/wiki/Tubing_(recreation)
The nearest thing to it would be para-gliding whereby you dangle at the end of a parachute towed by a speedboat, but watching a tubing video (link at the foot of the Wikipedia page), that might seem a bit tame in comparison.
But here's a site which lists all the festivals due to take place in Thailand which you might want to incorporate into your itinerary:
http://www.whatsonwhen.com/sisp/index.htm?fx=location&loc_id=131131
The Cambodian/Thailand border crossing at Aranyaprathet would enable you to take a train to Bangkok via the shortest route. Alternatively, you can compare the Thai rail network map @ http://www.railway.co.th/English/network.asp with other border crossings here: http://www.canbypublications.com/cambodia/overland.htm
But there have been some tensions between Thailand and Cambodia just recently due to a dispute of concerning a Hindu temple, see: http://www.time.com/time/world/article/0,8599,1850442,00.html so it will probably be wise to check in advance before you decide where to cross the border.