More ideas...

been thinking of some too...

1. Chimneys. we could look into the decine of them since the industrial age
2. Locks. Simliar to the letterbox idea and looking what lies behind them
3. Pipes/cables. As technology develops building need updating, items such as air con and the internet were not thought of when some building were built therefore they are often bolted on, unsightfully
4. Road lights. maybe we could look into the idea of light pollution and the effects of global warming
5. Letter or numbers. whoop whoop!