Any Suggestions for Indoor Activities in Berlin?

In the two forecasts below, one of these places in the middle of summer. The other is in the middle of winter. 

Glorious Berlin Summer

Rubbish Perth Winter

Vicky and I have some friends visiting from Perth this week. It's been piddling down pretty much the entire time they've been here. I'd like to blame them for the horrid weather, confusing the seasons, especially when last week was a little bit hot and I even wore shorts outside the house for the first time since leaving Dubai in January. I'd just be trying to stop myself from feeling so guilty, however, that their trip has been so disappointing. It's making it really hard to recommend afternoons in Prater, or Mauerpark, or the Tiergarten. Good one, Berlin.

Here is a glutenfrei donut

As mentioned, the weather in Berlin in the weekend just past was superb. Vicky and I decided to celebrate by heading out to West Berlin, buying a fancy picnic lunch at KaDeWe and eating a fancy picnic lunch in the Tiergarten.

Now, get ready to rub your eyes in disbelief because we also discovered a glutenfrei Berliner to eat for dessert.