Mr. Kipper and I are traveling separately from EWR to PRG, via ARN. I'm pondering two different flights, both through ARN. One has about a 5 hour layover, the other about a 12 hour layover. I'm leaning towards the 12 hour layover, so I have some time to venture into ARN for a bit.

How much travel time should I allow to get to/from the airport and back to the gate?
What should I definitely see in ARN?
Is it crazy to try to see anything in 12 hours? If it is, then I'll book the other flight.