Glad you found the info helpful.

I haven't been to Koh Samui (yet) but others on here have and can give you a lot of good advice. Look for posts from jaxw - she's been many times and has posted a lot of good info.

If you're going in April, you may experience Songkran - Thai water festival which ranges from gentle sprinkling to full-on drenching. While in a lot of places it's on a voluntary basis, in Chiang Mai, it tends to apply to everybody whether they want to be in it or not.... full-on drenching with everything from water guns and hoses to buckets scooped from huge drums of water. Some call it fun but I thought a lot of it was quite dangerous and stupid.
Chiang Mai is an interesting place though and a gateway to other places in the region.

Kanchanaburi has a lot of things to see and do. As well as the things already mentioned, there are waterfalls, bamboo rafting (gentle,not scary) or long-boat trips along the river, elephant rides and the World War 2 site "Hellfire Pass" - part of the Thai-Burma railway. If you're Australian you'll find this place especially poignant.

THere are lots of beaches and islands - some more glam than others (and prices across the range). You'll find something to suit.

If you feel like an adventure and want to go a bit off the beaten track, you could catch an overnight train and go up to Nong Khai - a large proviincial town on the Mekong. Take a tuktuk to visit the amazing (HUGE) stone statues in the special park just out of town,shop for silk, woodware,souvenirs and other bargains at the market and get a bit closer to the "real Thailand" . It's becoming a bit more gentrified/upmarket though so might not retain its provincial town charm for all that much longer.

Posters on here are very generous with their time, so if you have specific questions just ask and someone will be sure to answer.