I agree with Jeremy - Cameron Highlands sounds like a good place for you. Also Taman Negara, which is a big rainforest natural park, loads of outdoorsy stuff to be done there. I'd say its easy enough to organise your own transport there from KL when you arrive - going there with a tour probably isn't likely to give you enough freedom for all the exploring you sound like you'd want to do!
I also went over to an island in the South East of Malaysia - Pulau Tioman, which was ridiculously good for diving/snorkelling, and there were a couple of cross-island hikes to waterfalls/beaches etc that were quite intense. I also hear the Perhentian Islands are spectactular.
Good luck, I think you'll love it over there
