To get high, most commonly by means of marijuana. Trees is a common slang name for marijuana. When you climb a tree, you're getting higher in elevation than when you were on the ground. Therefore climbing trees refers to getting high off of trees.

IM CLIMBING SOME TREES TONIGHT. IT WILL BE COOL TO BE RIDING WITH YOU AND TREES. CLIMBING TREES MEANS THAT YOU AND I ARE GOING TO SMOKE THAT SWEET SATIVA TOGETHER.