In a former bus depot near the Viking Line terminal, Stockholm’s charmingly old-school transport museum is an atmospheric spot to spend a rainy afternoon. An impressive collection of around 40 vehicles includes several very pretty antique horse-drawn carriages, vintage trams and buses, and a retro tunnelbana carriage (complete with original advertisements). Kids can play tunnelbana driver (there's video from the driver's seat). Displays about the construction of the tunnelbana system starting in 1933 are pretty mindblowing.
The museum shares a space and entrance with Leksaksmuseet (one admission covers both).