Do everything! Get involved in all of the optional activities (or at least most of them, some like horse riding you can skip!), make sure you have enough for ALL of the safaris, especially Okavango Delta and Serengeti Park, and just get involved with the group! I did the big 56 day trip and it was amazing - I was travelling alone and met some of the most awesome people ever! It was different to a Topdeck trip as our guide was more there to make sure everything ran smoothly, not to tell us about history and stuff, but he was still super helpful. Plus we had to pitch in with cleaning the truck and stuff, but it was all evenly shared and a great experience. Just make sure you take plenty of sun screen and mosquito repellant! Oh, and a comfortable sleeping mat as mine was rubbish!