Replace the activity, not the whole day
When rain changes an outdoor plan, choose one indoor stop you actually want to visit. The National Museum is an option for a history-and-art-focused afternoon, subject to current opening arrangements. A long meal or a confirmed exhibition can also work.
Avoid turning the backup into a rushed circuit of indoor venues. You still need time to travel between them.
Check the door-to-door experience
An indoor attraction doesn't make the whole outing sheltered. Ask about drop-off, the walk from parking and whether the area you want to visit is open. Carry belongings in a bag that can handle the journey.
If rain is heavy enough to make travel impractical, staying where you are can be the right choice. A saved booking idea is not an obligation to set out.
Use a smaller objective
At a museum, pick one collection or theme. At an exhibition, read about the artist before going. At a restaurant, choose somewhere suited to a longer stay rather than occupying a busy table beyond the venue's expectations.
Save the National Museum as a possible alternative in your plan. Check the forecast and venue directly on the day, then tell everyone which version of the outing is going ahead.