The slice of cake westwood gave is pretty much what you asked for. 2 weeks isn't a lot, so after a few days in Bangkok, you will need to go to an island in Southern Thailand for the full full-moon party experience (timing will be necessary). You could try Koh Samet or (dare I say it) Pattaya for the full-moon party to avoid the long trip down to the southern islands. Then to Chiang Mai for the elephant experience (research here for a place that is more ethical about the way they treat the elephants) and maybe a day or two in a nearby place (Chiang Dao, Chiang Rai, Chiang Saen). Then head to Chiang Khong to cross into Laos. If you are doing it all by land then it will be rushed and an island plus Chiang Mai maybe all you can fit in. A flight (southern Thailand to Chiang Mai?) would give you more time.
I can flesh it all out a bit by saying that if you choose Koh Samet or Pattaya for full moon, then you may have time for a day or two in Sukhothai on the way up to Chiang Mai. You could try full-moon in Pai, near Chiang Mai. I'm not sure how worth it a party here would be and it's not an island/beach experience, but it is backpacker oriented and I would assume doing full-moon parties. Personally, I didn't like the place. This would avoid the long trip to an island or Pattaya to give you more time. Still very rushed for me, but some people like fast travel.