@Mor - In my experience it's important to plan, but also to pack as lightly as possible, especially if you are going to be hiking or traveling away from a hotel room. No matter how great your pack is, it's going to feel heavier and heavier and more and more pointless the further you have to carry it.
My favorite adventure travel tour was one in which I joined a group that explored California on a bus that converted into beds overnight. It was fantastic but space was at a premium and I quickly wished I had been less eager to bring every little thing I thought there was a distant possibility I might need.
Don't leave behind anything vital, but you don't have to bring the kitchen sink either. Particularly if you're going to be with a tour that will go past shops every now and then. You can always buy more bug spray or whatever else.