Nicki - we stayed 3 nights in Bangkok when we got here, which the kids loved. We did Koh San road, and went to a mall to the north of the city with a waterpark on the roof called Fantasia lagoon which was great, and empty. The mall had some fab shops too and was really cheap.
Then we came down to Koh samui. I planned just to stay the whole time on samui, but we made friends with an English couple who had a dd the same age as dd2, and ended up going to Koh phangan with them, which was lovely and I wish I could have spent more time there. I got really lucky with the first place I stayed, it was through air bnb, and the hosts (English man and his Thai wife) were so lovely and went out of their way to make sure we were ok, gave us lifts places, showed us around the island, and even after I moved on to stay somewhere else they invited me for a night out to watch the boxing (and arranged for their housekeeper to come and babysit), and a dolphin spotting and snorkelling tour to a conservation island.
I also made friends with an ex pat who lived at the first place I stayed, and we've met up a couple of times for drinks, and I've met some of her friends. We also got to know a Dutch family who came on the dolphin tour and they were lovely.
We've done lots of hanging around on the beach, I hired a car for 8 days so we had days out, pretty much every day we've been meeting up with people that I've got to know out here, my social life here has been way better than at home! I think that's what has made it so good, I've made some good friends who have made a real effort to include me in things, knowing that I'm on my own with the dc.
Can give you the name of the first place I stayed if you like, I highly recommend and it was only £20 a night!