An abandoned house near my friend's house in Monaghan where i'm from. There were two dead and almost fully decomposed mice just lying on the stairs and on the floor, like they had died of old age rather than anything else. Then we went upstairs, with difficulty, as they were partially collasped, and there was still clothes hanging on an inside line across the room, long johns and all. Pair this with the fact that it was dusk and the house was surrounded by trees, and you've got some pretty good scope for imagination. Needless to say it went wild and we ran from the house pretty terrified.
There was also a windowless room that we dared each other to go into, but the light from your phone doesn't exactly inspire confidence. Good Fun though, in a way.