I will be spending a month in Thailand and want to know if I will be spending my time wisely, since this will be my last stop on my 3 month trip to SEA I want to spend a lot of relaxing time at beaches (including learning to dive in Ko Tao ), as well as some trekking in Chaing Mai, and at least at one point during my trip run into some elephants.
I was thinking of taking the time I've put for Ko Samet and adding it somewhere else, cause I will already be spending a lot of time at other beaches and it just seems like a lot of time that I could be using else where figuring as I just wanted to utilize the last few days to stick close to Bangkok.
Pnom Penh > (1h Plane) Bangkok [21st] Bangkok > (12h Sleeper Train) Chiang Mai [22nd, 23rd, 24th, 25th, 26th]
Chiang Mai > (12h Train) Bangkok > (8h Sleeper Train) Chumphon [27th]
Chumphon > (3h Boat) Ko Tao [27th, 28th, 29th, 30th, 1st, 2nd, 3rd, 4th, 5th, 6th]
Ko Tao > (2h45 Boat) Ko Samui [6th, 7th, 8th]
Ko Samui > (h30 Boat) Ko Pha Ngan [8th, 9th, 10th(Full Moon Party), 11th,]
Ko Pha Ngan > (1h30 Boat) Ko Tao > (3h Boat) Chumphon > (8h Sleeper Train) Bangkok [13th]
Bangkok > (3h Bus) Ban Phe > (1h Boat) Ko Samet [13th, 14th, 15th, 16th]
Ko Samet > (1h Boat) Ban Phe > (3h Bus) Bangkok (17th, 18th]
Bangkok > (1h30 Plane) HCMC > Home
You plan looks fine based on the things you've said you want to do. I'd recommend booking your sleeper train travel as soon as possible to make sure you get the dates and times you wish. Rather than taking the 12-hour train ride back to Bangkok from Chiang Mai , I'd fly instead, particularly if you are just getting on another train for the ride to Chumphon. If it is still an option, you might consider a flight from Chiang Mai to Don Muang (rather than Swampy), as that will put you on the train line for an easy ride to Hualamphong train station and the connection to Chumphon.
Also, agree that making your way to Ko Samet won't be your best option. Why leave a great beach and spend the time and money just to get to another beach that may not even be as good as where you are? Just spend those extra days and money where you are on the gulf islands. If you wind up with extra time at the end of the trip, there are always places like Kanchanaburi and Ayutthaya as quick and easy trips out of Bangkok before flying to HCMC.
Hope that helps. Have fun. Cheers.