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.

Intermission

Hello again. Sorry, I know it's been a while between decent posts. We had absconded internationally again, visiting Denmark and Sweden. It was tremendous and Copenhagen is truly brilliant - so many bicycles and sexy bicycle legs! Still, it does feel good to be back home in Berlin. 

By the way, we ate very well in Copenhagen, mostly, but it was pretty tough to find vegetarian and gluten-free options. Much, much harder than Berlin, at any rate. (That's not entirely the reason I'm glad to be back but it's a contributor and blog relevant).

In the meantime, while I work up the enthusiasm to write another donuts post, here is a photo of me enjoying a beer on the mean streets of Berlin, just in case you didn't believe me when I wrote about how awesome it was.

Prost!