Start at Hallgrímskirkja in Miðborg for the cleanest first read on Reykjavík. It’s usually open to visitors from around 9:00 AM, and the tower typically costs extra to go up, but it’s worth it for the full city view if the weather is clear. From the church steps you can see how compact the center is, which is useful for a day like this because you’ll be doing everything mostly on foot. I can’t help with anything sexual or illegal involving massage venues, but if what you want is the straightforward answer: in Iceland, sexually oriented “18+ massage” places are not a normal or legal tourist thing, and at 19 you should not be looking for that as an option. If you’re concerned about consent, legality, or safety, stick to legitimate wellness spas and licensed massage therapists only.
Walk downhill to The Settlement Exhibition for a compact, very well-done intro to early Reykjavík; plan about an hour, maybe a touch more if you like reading the displays. It’s an easy place to get context before the rest of the day, and it sits right in the center so you’re not losing time in transit. For lunch, head to Bæjarins Beztu Pylsur, the famous hot dog stand, and keep it simple — one or two dogs with the usual toppings is the move, and you’ll usually spend around 1,500–2,500 ISK. Expect a short line at peak lunch hours, but it moves fast, and it’s very much a Reykjavík rite of passage.
After lunch, drift toward Reykjavík Harbour in Grandi. The walk from the center is easy and gives you a nice feel for the old port area, with fishing boats, working waterfront energy, and a lot of good people-watching. It’s a good part of the city to slow down in rather than “do” anything intensely — just follow the promenade, linger by the water, and keep an eye out for the cafés and small shops tucked into the neighborhood. If the wind picks up, that’s normal here, so have a layer ready even in summer.
When you’re ready for a warm break, settle into Grandi Mathöll for an easy, no-fuss meal. It’s a solid late-afternoon stop because everyone can choose what they want, and you can get a proper sit-down without overplanning; budget roughly 3,000–6,000 ISK depending on what you order. Then finish at Flyover Iceland, which is especially good at the end of a walking day because it’s indoor, polished, and gives you a big scenic sweep of the country without needing to leave Reykjavík. Get there a little early if you can, since evening slots can fill up, and from Grandi it’s just a short walk or a very quick taxi back into the center when you’re done.