Not been to Laos yet, but is highly recommended by friends.
As Trebor says Thailand is a trip all on it's own. However, it is a great starting off point for a trek, particularly if you are on your own. Bangkok is cheap to fly to (airasia.com for cheap flights all round SE Asia) and Ko Sahn Road is full of backpackers, bars, restaurants and travel agents. Chiang Mai or Chiang Rai in the north are good stopping off points for a trek into the jungle, ride elephants, bamboo rafting and stay with the hill-tribes. From there, you can travel overland into Laos (I have not done this).
I would add Tonle Sap lake -the largest in SE Asia-to Trebors list for things to do in Cambodia if you make it there. A trip can be done in half a day from Siem Reap, or can get a boat across it from Siem Reap to Phnom Penh.
Bangkok/Chiang Mai and Siem Reap are all good nights out.
Sadly, I cannot join you on the tour as heading into China for a trek and am not young either
PM me if you want any more specific advice on Thailand or Siem Reap.