surveillance robots in urban situations

My friend runs a great web site all about robots here
He is looking for ideas about how to make a surveillance robot work in a urban environment.
This is for the MOD grand challenge

Idea 1
Use a small hot air balloon painted black to float onto rooftops at night, deflate the balloon during the day, put a little camera onto it so that it can take pictures.

Idea 2
Hide a robot in a football or coke can and it can roll around the city listening to people and taking pictures.