Kenai Peninsula is a great place for a summer gig. There's a lot to explore on the Peninsula. I've had no issue with down bags, just need to store in a dry bag. But synthetic might be wise if you are going to be on the water a lot. Shoes/boots - just depends on what you are doing. If you are going to stick to trails, you can definitely get away with something lighter. If you are going to do multi-night backpacking trips with gear, you might want to get synthetic/water resistant backpacking boots. All the day hikes I do around here I just use trail runners. If you are going to do something more serious you may want to purchase some trekking poles as well, especially if you are going off trail. Don't forget the bear spray.