There is a Reykjavík weather pattern where it rains for forty minutes, clears for ten, hails for five, then rains again. On those days you need indoor moves that absorb three or four hours without anyone having to keep checking the radar. These are the rooms that work.
The Nordic House library
Alvar Aalto designed the Nordic House in 1968 and it still feels like the most calmly Scandinavian building in the country. The library on the lower level is small, quiet, has a kids' section that is charming on a Sunday, and the cafe upstairs makes good cardamom buns and proper coffee. You can spend two hours there reading, drawing, or just sitting near a window watching the weather hit the marshes by the university. It is a fifteen-minute walk from downtown and almost no visitors find it.
The Reykjavík Art Museum: three buildings, one ticket
One 2,100 ISK ticket covers Hafnarhús by the harbor, Kjarvalsstaðir in Klambratún park, and Ásmundarsafn in Laugardalur, valid the same day. Hafnarhús has the Erró collection plus contemporary shows in big concrete rooms. Kjarvalsstaðir is for landscape painting, especially Jóhannes Kjarval, who painted lava in a way that makes you understand the country better. Ásmundarsafn is the smallest and weirdest, a sculptor's domed studio house turned museum.
Doing two of the three in a day with a coffee in between is a real date and easily fills four hours.
Whales of Iceland in Grandi
This sounds like a tourist trap and is, but it is a good tourist trap on a rainy day. A warehouse in Grandi with twenty-three life-sized models of whales hanging from the ceiling, dim blue lighting, ambient whale-song audio. It is bigger than you expect inside. Tickets are around 3,500 ISK. Spend an hour, then walk one block to Valdís for ice cream, which Icelanders eat in any weather.
Bíó Paradís plus dinner across the street
The independent cinema on Hverfisgata is the rainy-afternoon move. Check the schedule for a film with a runtime that lines up with dinner across the street at Skál in Hlemmur Mathöll, three blocks east, or at Hverfisgata 12 for pizza. The cinema bar is also a fine place to extend the date if neither of you is in a hurry.
Sundhöll Reykjavíkur, the indoor-outdoor pool
Most Reykjavík pools are mainly outdoors, which is fine but cold to walk between in a downpour. Sundhöll on Barónsstígur has a proper indoor 25-meter pool plus the rooftop hot tubs, so you can warm up indoors and only step into the rain when you actively want to. The Guðjón Samúelsson architecture is half the appeal. Entry around 1,330 ISK.
The bookstore-cafe loop
Mál og Menning on Laugavegur has a full cafe upstairs and lets you sit indefinitely. From there, walk three minutes to Eymundsson on Austurstræti, which has another cafe and a bigger selection. From Eymundsson, five minutes to Mokka Kaffi on Skólavörðustígur for the waffle with jam and cream. Three places, three coffees, three hours, and you have spent maybe 4,000 ISK total while staying mostly dry.
The Settlement Exhibition
Underneath a building on Aðalstræti there is a Viking-era longhouse foundation that was excavated in 2001 and turned into a small museum called the Settlement Exhibition. It takes about forty-five minutes and is a good prelude to lunch upstairs at the Reykjavík Roasters two blocks away. Tickets around 2,200 ISK, and your ticket also gets you into the larger Reykjavík City Museum at the harbor on the same day.
Kolaportið flea market on weekends
Kolaportið is open Saturday and Sunday from 11 to 5, in a warehouse by the harbor next to the Customs House. It is the only flea market in the city. There are old vinyl records, used books, woolens, dried fish that smells assertive, and one stall that sells fermented shark if you want to dare each other. Free entry. Easily an hour, more if you actually buy something.
The long lunch as a tactic
When the weather is genuinely impossible, commit to one place for two and a half hours. Snaps Bistro on Þórsgata is the best room for this, glassed in like a winter garden, and the lunch menu runs through the afternoon. Order the moules frites, share a bottle of wine, and let the rain do whatever it is going to do outside. Around 6,000 to 8,000 ISK per person including wine, which is mid-range for Reykjavík and worth it for the four-hour shelter.
What does not work
The Hallgrímskirkja tower on a rainy day, because the view is the whole point and the view is gone. The harbor walk in heavy wind. Anywhere that requires walking between two buildings more than five minutes apart, because Reykjavík rain is rarely vertical and umbrellas die fast. Plan tight, stay dry, and let the weather be the date's third character.