What are some fun activities for 18-21 year olds to do in Gatlinburg?

Omg!!! You’re going to have so much fun! I’m jealous!
Now, to answer the question, I’m around the same age as you, and I actually love going to Gatlinburg and Occoe with my family. We like to go to
1) Ruby Falls,
2) the Aquarium (It’s huge! If it sounds boring, okay, pass it up, but it can actually be pretty cool if you’re into that kind of thing)
3) Go hiking in the smokies (we’ve seen bears up close! scary, but fun!) If you can time it to be on top of a ridge at sunset, you won’t be disappointed. it is a magnificent experience
4)You need to eat dinner at one of those restaurants with patios overlooking the river, too! The views are gorgeous!
5) There’s also Dollywood. I haven’t been there since I was a kid, so I’m not sure if I recommend it, but some of my friends had fun there last year.
6) Also, wandering around “downtown” near the Aquarium and the Imax is actually really fun. There are funky cafes, used bookstores, and that type of thing.

Sex is always fun for people of that age.

I went there for my honeymoon (I was 21). There is so much to do! The Guinness World of Records is a must see, hiking, helicopter rides, horseback riding, OberGatlinburg has an aerial tramway that will take you to an area that has an ice skating rink and alpine slide. There are also some very interesting caves in the area that you can visit. Have fun!