In Chiang Mai I recommend Mautour Services for a trek. They've been in operation for 30 years (the son took over after the father started the business in the 70s).
It's one of the companies that actually organize their own treks, in opposition to agencies just selling them. Might not be the cheapest around, but you'll have the quality.
For a beach, there are some quiet ones on Koh Samui, like the Tong Takien Bay between Chaweng and Lamai. Samui is perfect for people looking for both quiet spots and animation. Renting a bike is recommended, but you'll have to drive carefully - no speeding (sandy roads can be treacherous especially when taking a curve).