Sounds like a fun (and warm!) day! :D
I haven't tried Ingress, but doing it together with others sounds like a good way to game outside.
I've been to Cologne a couple of times and seen the beautiful Cathedral, a bit creepy and majestic at the same time. It's like Dark Souls in real life!
That's exactly the right desciption of the cathedral! It was really impressive and the first time I saw it :-)