A long indoor date needs a room that lets you stay. Most cafes in Sliema will tolerate ninety minutes if you keep ordering. A few will let you sit for three hours and bring you water without asking. These are those places, plus the museums, shops, and activities that work as a full afternoon.
Agenda Bookshop at The Point
Agenda is a proper bookshop with a strong English-language section, a decent local interest shelf, and enough space to wander for an hour. The date move is to give each other ten minutes to pick a book the other person should read, swap, and then go for coffee at Cafe Cuba downstairs and explain the choice. It sounds small, it lasts two hours, and you learn more about each other than you would over dinner.
MUZA in Valletta
The ferry from Sliema to Valletta runs in most weather. MUZA, the national art museum, is in the old Auberge d'Italie just up from the ferry landing. Entry is around 10 euro. The galleries are arranged thematically rather than chronologically, which gives you actual things to discuss. The cafe in the covered courtyard is good for a long coffee after.
If MUZA does not pull you, the National Museum of Archaeology on Republic Street is ten minutes away and houses the Sleeping Lady from the Hypogeum. That alone is worth a slow hour.
A Long Lunch at Ta' Kris
Maltese rain is the right weather for rabbit stew. Ta' Kris on Fawwara Lane in Sliema does the proper version. Book the 1pm slot, order a bottle of wine, take three hours. The dining room is small and warm and they will not push you out.
The Point Cinema
Eden in St Julian's is the bigger venue, but the Point in Sliema has a smaller cinema downstairs that is fine for a matinee. Pair it with the bookshop trip above and you have an afternoon. Tickets around 8 to 10 euro.
Fortina Spa
The Fortina on Tigne Seafront takes day passes for the spa and hammam. It is not cheap, around 70 euro a head for a couples package, but it is two hours of warm rooms while it pours outside. Book ahead. This is more committed-relationship than first-date territory.
A Cooking Class
There are a few small operators running pastizzi-making and traditional Maltese cooking classes around Sliema and Valletta. Three hours, around 60 euro a person, and you eat at the end. The format does the work of date conversation for you because you have something to do with your hands.
Cafe Berry, the Long Sit
Cafe Berry on Tower Road is the cafe where you can sit for three hours on a rainy afternoon and nobody minds. The coffee is good, the pastries are honest, and there is enough background activity to feel like you are part of a city without being interrupted. Bring a deck of cards or a book to share. This is an underrated move.
The Sliema-Valletta Walking Tour, Indoor Version
If the rain is properly heavy, do Valletta's covered routes. Republic Street is partially arcaded, St John's Co-Cathedral is essential and warm inside, and the Casa Rocca Piccola gives you a tour of an actual lived-in palazzo with a guide who has personality. String those three together with coffee at Caffe Cordina on Republic Square and you have a five-hour date that barely gets wet.
Manoel Theatre Tour or Show
The Manoel in Valletta is one of the oldest working theatres in Europe. They run building tours during the day and shows at night. A tour is around 5 euro, a show varies. The interior is worth the trip on its own. From Sliema, ferry plus ten-minute walk.
What Holds Up and What Doesn't
Things that hold up for a long indoor date in this city: bookshops, museums, restaurants that do not turn tables, spas, and cinemas paired with something else. Things that don't: shopping malls treated as the whole plan, cafes you have already sat in for an hour, and any plan that requires walking outside between stops in real rain. Pick one anchor, one secondary, and one cafe. That is the formula.