In reply to bryn keating: I've been to Thailand, Cambodia and Laos during the summer months. Thailand is the hardest to get off the beaten track, heavily 'touristified', but it can be done.
Cambodia it's quite easy to get off the beaten track, Laos is even easier. In my experience, the less touristy the country was, the more enjoyable it was. Laos was easily my favourite country out of the three. It's the least developed and it's got good trekking too. I think in terms of the scenery, it's all very similar in all three countries, so personally I'd go for the country that is the best aside from the trekking i.e. Laos.
The only thing I'd recommend Thailand for is scuba diving on the islands and/or west coast and going to Chiang Mai, hiring good motorbikes from Tony's Big Bikes and doing a roadtrip.