Things to do in/from Positano with 18 year old

Vietri sul Mare is known for ceramics, but there are also some shops in Ravello.
use ferry whenever possible: https://www.travelmar.it/en/index
One day for hiking, one day for Capri, and one day for village hopping by ferry should be nice!
I'd try to do lunch in Cetara.

Other than 2 clubs, Music on the Rocks and Africana, just wandering the town and some nice restaurants and bars is all you’ll find.

https://search.ricksteves.com/?button=&date_range=2y&filter=Travel+Forum&query=positano+teen&utf8=%E2%9C%93

Becky, unfortunately I don't think you're going to find much for evening activities in Positano. It's really small, mostly uphill, and doesn't offer much outside of hanging out and a bit of shopping even during daylight hours. Let's just say that it wouldn't have been my choice with a young adult but I'm sure you can get creative, and it does have a few beaches, albeit more rocky than sandy so bring water shoes!

https://www.positano.com/en/e/the-beaches-of-positano
https://www.positano.com

Capri offers some nice hiking around the island, and I'd choose to do the chairlift from Anacapri up to the top of Mt. Solaro over the Blue Grotto (which is what I think you meant by Blue Lagoon). IMHO if there's a tourist trap on Capri, that's the one. The hike (actually more of a walk) up to Villa Jovis is just one nice one of what's on offer. Anyway, spend some time with the website for ideas, keeping in mind that while it touts a lot of tours, you don't need them if willing to do some reading up. We spent a couple of nights there and did all of our exploring on our own. It would be pretty easy to spend the lion's share of a day there if you're interested in what's there to see and do, but you won't be able to stay into the evening as the ferries don't run that late; last one to Positano looks to be at 6:00 PM.

https://www.capri.com

Ferry schedules around the region; used the departure/arrival dropdowns to see routes:
https://www.positano.com/en/ferry-schedule

Any interest in Pompeii?

Thanks for the ideas! This leg of the trip is just R&R or as Rick says "a vacation from our vacation." I love the idea of hiking one day, exploring AC villages by ferry one day, and Capri one day. Any advice/fun detours/surprises exploring AC by ferry? I know we should do this on the best weather day. Thanks for the tips on Capri. I feel like the blue lagoon is a must do, just once. Any advice on best tours to the lagoon?

Blue Grotto (which is what I think you meant by Blue Lagoon)

you won't find tours for a "blue lagoon"

We were on the Amalfi Coast last month and had also come from Florence. Even in May, it was crowded and hot. We stayed outside of Amalfi. Positano and Ravello were our day trips. Ravello in the morning was lovely. Positano was packed and seemed more expensive than other places. There was a part of me that wished we could be there after the daytrippers left. We hit Pompeii on the way out. I would have like to spent some time in Salerno.

I'm not entirely sure what would please an 18 year-old. I hear there are small beach resorts that you can book for a day at the beach and watersports. Maybe you could rent a boat or get on a boat tour besides Capri. I did like riding on the ferries. I didn't see much like jetskis or tubing which are water activities my kids like. I heard raves and groans about the Trail of the Gods. All the ceramics places looked pretty much the same and I never found many artisans. Maybe you can find a place to tour where limoncello is actually made. That would have been interesting.

I know this sounds negative but we did enjoy our time on the coast. This was towards the beginning of the third week of our trip and a quarter million steps into it, we needed some R&R. We had a lovely Airbnb overlooking the Atraini beach with a killer view of the sea. We had slow mornings and lingered over our coffee. We spent a lot of time reading and drinking wine on the deck. At this point, we were tired of restaurants and enjoyed stopping at the market to find things to cook. Some mornings, there was a little van that sold fresh fruit and veg in the neighborhood. Interesting meals can be kludged together in an apartment.

Everybody vacations differently. We took our time on the Amalfi Coast to sleep in (after viewing the sunrise) and not feel pressured to go places. Others need to keep moving. I hope you find a happy medium for you.

Any advice on best tours to the lagoon?

Right. As Christine said, it's Blue GROTTO, not lagoon. Info:

https://www.capri.com/en/s/la-grotta-azzurra

Note that those boats only spend about 5 minutes inside (if the seas are calm enough to even do that), the wait in line-up to get in can be long, and it's not unusual to be aggressively hassled for tips. We skipped it and didn't feel like we'd missed anything but YMMV.

https://www.tripadvisor.com/Attraction_Review-g488299-d195537-Reviews-Blue_Grotto-Anacapri_Island_of_Capri_Province_of_Naples_Campania.html

HI Becky! The Amalfi Coast is one of the top places I've visited so far! It is stunningly beautiful! So we are returning for the 4th time, in just a few weeks! It will be myself, my husband and our 20 year old daughter. We are staying in an apartment in Positano (booking.com) for a full week from July 8-15. Here our some of things we plan to do while in Positano:
(1) skip the main beach that all the tourists use, walk around the right side of Positano (facing sea) to Fornillo Beach. It should be less crowded. There are restaurants and bathrooms, etc. You can ask for directions to the path to Fornillo when you arrive.
(2) If we want to use a pool (with a view,) we are going to buy a pool day pass at the Posieden Hotel. I've read this is possible. They also have what sounds like a very nice restaurant and rooftop bar you can go to.
(3) I want to take the local SITA bus to a town called Furore, which has a gorgeous beach, just google Furore. We'd like to spend a few hours on that beach!
(4) We may take the SITA bus in the other direction to spend an afternoon in Sorrento, since we've never actually checked out Sorrento and we have the time.
(5) you can take the ferry to Capri for a day trip. We've been to Capri several times, so we are skipping it on this trip; instead after the Amalfi Coast we are going to stay on the island of Ischia for 4 nights.)
(5) If you want to buy ceramics, take the SITA bus to Vietri Sui Mar. That's where you will find the best prices. We have brought home ceramics from the shops here, or they will ship them to you.
(6) the best thing we ever did on the Amalfi Coast was to rent one of the rubber motor boats to motor along the coast at our own pace. We stopped and swam in a little waterfall; we motored into a big cave (had it to ourselves.) Maybe you can find another couple of people who would want to split the cost with you/drive the boat if you are not comfortable motoring it yourself. It really was amazing!
After, Positano, we are spending 4 nights in Ravello. Why? Because I adore the two amazing, large, gardens there (Villa Rufulo and Villa Cimbrone.) I plan to do each garden on separate days. I also plan to visit Amalfi from Ravello. Ravello sits way above Amalfi, about a 15 minute Sita bus ride.

Anyway, if any of this sounds enjoyable to you, do visit the Amalfi Coast.

Any advice/fun detours/surprises exploring AC by ferry?
In Minori, you have Pasticceria Sal De Riso, and there is a Roman villa. http://www.ciaoamalfi.com/2019/08/villa-romana-minori/
I think some of these "lesser" sites might make for a nice contrast to the mobs of day trippers in Positano and Amalfi.

Oops! Yes, of course blue GROTTO, not lagoon. Thank you for all the great tips here!

Kerry, Thanks for the amazing list. LOVE the idea of renting a boat ourselves; my husbands dream! Do you have a recommendation for where to do this?

We rented from Banana Boats on Capri on 2 separate trips. Super fun. They were real boats, not rubber, small, very nice, had a canopy, comfortable seating, an anchor, a ladder to get in and out for swimming, easy to drive, and reasonably priced at the time. They’re located to the right of where the ferries dock.

For some interesting walks, the 3 below I wouldn’t consider hikes. Good way to get out of the heat in some cooling shade.

https://www.tripsavvy.com/best-hikes-on-the-amalfi-coast-4779532

#s. 2, 7, 9.

For boat rental.

https://www.lucibello.it/en/boats?page=1

Go to page 2.