Every year, on May the 17th, Lisbon and the rest of Portugal celebrate the Museums Night with music concerts, guided tours and whole program of fun activities.
The program is vast, and can be consulted here. I know that it’s only in Portuguese, as this night is mostly aimed at locals, but please feel free to use the comment or email box, shall you need any help or suggestions.
There are plenty of fun activities to choose from. Theater plays, staged tours, organ concerts, dance sessions and also activities for children. If you prefer to visit a museum on your own, just pick one, as all the museums will be open until 11pm, with free admission.
The next day is celebrated as the International Museums Day and it’s also full of activities, guided tours and free entrances. I know that the sun is shining out there, but if you’re looking for something indoor, there’s a lot to choose from!