First: bring a wildlife book or guide of some kind! And generally bring your curious spirit. There's SO MUCH you can learn just by asking questions out there and looking for the answers, and it's totally worth making the extra effort to understand the landscapes around you.
Second: you're going to spend a lot of time with a small group of people. Be open to them all during the whole semester. It feels great to be given a clean slate during the semester, and even better when you can reciprocate that for everyone.
Last, and perhaps most important: This is truly a once-in-a-lifetime kind of experience. Approach it every day with gratitude and wonder and an open heart.
The wilderness and conservation semester (HMI's heaviest backpacking semester) is essentially a series of 2-week-long expeditions. In the backcountry, most days mean waking up, cooking breakfast on the WhisperLite and packing a lunch, and then packing up camp to leave for a hike to the next campsite. When you get there, you set up camp, hang out and explore depending on the length of the hiking day, cook dinner, and then turn in for a night under the stars, usually exhausted and incredibly glad for the warmth of your sleeping bag. There are usually some layover days where we do day hikes or play group games sprinkled into the expeditions. Then, between expeditions, we stay in a hostel, clean our gear and get it ready for the next expedition, reconnect with family and friends, and rest up.
I was quite nervous about spending so much time with so small a group, and also about the climbing portion of our semester.
I shouldn't have worried about the group - the community is facilitated well by instructors, and I think HMI attracts interesting, kind people so we really got along. There were obviously some moments of conflict, but we worked through them, and if I needed time alone I could usually take a moment when we got into camp to journal or nap or whatever I needed.
In terms of the climbing, I felt like I was incredibly good hands - the HMI staff are really qualified and competent and great teachers, so the climbing portion ended up being for me a real chance to conquer some fears.
You should bring small games and things like that. Playing cards was a huge icebreaker for us, and playing party games like Salad Bowl and others was a lot of fun.
Bring plenty of bandanas - you'd be amazed at the multitude of uses they have!
Bring books related to the places you'll be in - I thought it was totally worth the extra weight to be able to read about the landscapes we trekked through.
Bring a packable daypack - the bigger the better! We use them for all kinds of things, and being able to fit more stuff in them is very helpful...