In Germany, there are many observation towers, which can be visited 24 hours per day without paying a fee. There is also no person supervising the visitors of the tower, so you could do in principle many possibly illegale things there. ( If you expect there many griffities and vandalism. It is not so. You can find there some smaller graffities on the railings, but only rarely larger ones )
Such towers are for example
Lemberg Observation Tower (
http://skyscraperpage.com/cities/?buildingID=63852 )
Wanne Observation Tower (
http://skyscraperpage.com/cities/?buildingID=63853 )
Büchenbronner Aussichtsturm (
http://skyscraperpage.com/cities/?buildingID=59146 )
Are there such towers also in your country, which you can visit whenever you want and no-one is supervising you?