We used agoda for the centara and booked direct with most others, it depends who is offering what prices and packages! I think any overnight adventures might be best booked in advance (have a look at siam safari, limestone lake tours and elephant hills websites, for example), but day trips can easily be organised a day or two beforehand, either in your hotel or at the many many tour places around all your destinations. You do need to do a little price comparison- flight of the gibbon was cheapest booked via the centara tour desk but other things were more expensive. We also booked our cooking school before we left home, but this wasn't really necessary.

My husband is taking a sabbatical next january and will be in thailand for his 60 days, sadly, I can only join him for 2 weeks or so, but will make the most of it!