As someone who also loves the outdoors:

If I was buying a gift for an outdoorsy kid near Auburn it would definitely be related to rock hounding.

Auburn is right next to Mt.Apatite where they can go look for all sorts of cool rocks and minerals.

Maybe if you can find a guidebook on rocks and minerals of Maine and a shovel and some gloves? Also, if you want to go all out maybe they sell gift cards to the business "Dig Maine Gems" at Mt.Mica? In warmer weather the Sunday outings are a ton of fun and would really get them interested in exploring Mt. Apatite which is closer and free.

Honestly just writing this makes me want to go out there 😅

Edit: link to dig Maine Gems https://www.digmainegems.com/field-trips I don't think they open till end of March or early April