I would go to bangkok to experience it...night markets/shopping/food/usual temples/tourist stuff and that.
then koh samet where the locals go to avoid the usual tourist islands not a bad place...you'll have to pay an entry tax though
take a 12 hour train ride south to surat thani (I think - that's what the stations called)....some memories there
then please check out krabi and railay beach, aonang...
visit koh tao for diving and samui as others have said and ang thong (emerald lake)
there were a few more places I visited but here's a start.
if you want more details let me know.