I read lots of reviews before starting on the trip (very helpful), and one thing that gets mentioned a lot is those lockers! Padlocks: as others have mentioned, suitcase padlocks are too small. I had a 40mm padlock, which did the trick. The market in Musanze (Ruhengeri) sells locks if you start in Rwanda and still need one. I left mine on the truck for someone in the next group to use. If you don't particularly need it again, consider doing the same, as there's bound to be someone in need.
I had the impression that we were to empty our belongings into the lockers and store the holdalls elsewhere, but this was not so - the bags themselves also needed to go into the lockers. Mine wasn't quite squashy enough when full, even though I didn't think I'd overpacked. Our group was relatively small, so I was able to nab a second locker, which meant with the empty holdall in one, I could easily get at what I wanted in the other without taking a big bag out. Obviously you can't count on this, so do make sure your bag will fit into the given dimensions of the locker. The trip notes do say that sleeping bags and shoes can be stored elsewhere on the truck, but this wasn't the case on our trip. There is a roomy luggage rack above the seats where you can put things during drives, though of course this isn't secure storage.
I took several small dry bags which were invaluable, protecting things from water and dust. It rained every day in Rwanda and Uganda (October), mostly in the late afternoon, which was often around the time we arrived at camp. The waterproofs and walking boots got used well beyond the day of the gorilla trek.
The trek itself took place in the morning so we weren't affected by rain, and though it was a bit wet and muddy underfoot, it wasn't particularly hard going - especially when you had a porter to carry your bag for you, as I did. The porter I hired even helped me over the muddier/steeper bits, though I could have managed fine in truth. How far you trek depends on which gorilla family you visit and whereabouts in their territory they happen to be. Ours was about an hour and a half. They give you walking poles, so there's no need to bring your own. When you "find" the gorillas, you leave your bags and poles with the porters and just take your cameras. With hindsight, I'd advise keeping the gardening gloves at this point. Our Exodus group was split into two - maximum 8 visitors per gorilla family - and in the case of both groups we spent part of our time following the gorillas on the move. It was quite sloping and slippery with undergrowth that does its best to trip you up, and I did far more vegetation-grabbing than on the trek up to find them. Even the clearing where we were lucky enough to watch our family resting for a good while was on quite a steep slope.
After our gorilla encounter, my group was driven back to one of the park bases (not where we all started from in the morning) where there were a few souvenir shops. We were able to get some nice souvenirs: gorilla-related carvings, postcards and t-shirts of the "I tracked gorillas in Rwanda" variety etc. Our friends in the other group, however, didn't stop off there so had no opportunity to get any of these. I assume this place is only on the route back of certain gorilla territories, so it's a lottery as to whether you go there or not. If you're interested in such souvenirs, it might be worth saying "Get me the t-shirt!" to someone you won't be trekking with, in case they go there and you don't. That was the only place on the entire trip where I found postcards with gorilla pictures, and even the craft shop we found in Musanze (nice stuff but not cheap) surprisingly didn't feature gorillas much.
At Jinja we were able to get laundry done for us, and at most of the other campsites it was easy enough to do a bit of handwashing (taking a washing line is a good idea), at least at the two-night stops where there was time to get things dry. I was a bit disappointed with the stay at Jinja (though the tents were nice and the showers were good and hot). Those who went whitewater rafting had a great time (or so they said - I wasn't there to hear the screams of terror), but some of us opted for a jet boat ride, which we were told to book in the morning. When we turned up to do this though, the camp staff couldn't get through to the jet boat company (not sure how hard they tried), so we were left with a free day and not much to do, as by then it was too late to book anything else. A couple of us went for a swim at the hotel pool next door, where they charged a small fee. Just be aware that booking things from the campsite staff may be a bit hit or miss.
Money: when changing US dollars into the local currency, large denominations get you the best rates. We often found it easiest to tip in US dollars too, so $1 notes were also very useful, and I wished I'd brought lots with me as of course they come and go very quickly.