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.

Chilly Chilli

For a country with such a cold, grey climate, Germany does hot/spicy food surprisingly poorly. Rather than attempting to whisk away the winter blues with a lovely, warming curry that'll set your mouth on fire and run rockets down your gullet, the Germans appear to prefer sipping on lukewarm mush that matches the dreary sky above.

While Vicky and I have struggled in vain to find a decent spicy meal, no experience has so disappointed us as our evening eating 'hot chili' cornchips with a 'hot' salsa dip.