[edit] Things you should bring

Your passport or ID card
Your health insurance details
Enough cash or a Visa/MasterCard or the debit equivalents
Power adapter for(s) these plugs
Power strips, althought we do have to be careful with these at the venue to avoid tripping the fuse breakers. The dorm rooms might have only 1-2 power sockets.
Ethernet cable (WiFi is also available in the hostel)
Towel (can be rented for 2 €)
Soap, shampoo, shower gel
Toothbrush, toothpaste
Rain gear and a warm sweater — you never know…
Sunscreen, sun hat, sunglasses
Cheese and wine from your region for DebConf15/CheeseWineBoF
Earplugs (if you share room with fellow debconf participants... you never know who's gonna snore loud)

[edit] Things you could bring

Bikes, there's a basement room for safekeeping.
Balls, there's a volleyball court on site, and sports pitches nearby, which we may be able to use.
Frisbee, other toys, etc.…
Table tennis gear, if you distrust the stuff provided
Swimming gear, the river and an outside swimming pool are nearby
Your favourite poems, for the poetry night, at least ;)

Home-cooking devices including water heaters (against house rules)
Alcohol other than wine for the C&W party (it will be available from the bistro)
Food & drinks (they will be available from the bistro with reasonable prices on site)
- Baby food and food needed due to dietary requirements are obviously exempt