Rotherhithe tunnel in London screams to be repurposed this way -- to become "colorfully lit, art-lined" thing where psychologists give input on how to make it more welcoming
For example, a tunnel (or bridge!) between Canary Wharf and the Rotherhithe peninsula, replacing the existing passenger ferry, would have huge economic benefits by encouraging development of a relatively under-developed area.
The Greenwich foot tunnel is a great asset, and despite not being very cycle-friendly is very popular with cyclists too.