As Simon G says, just go for a wander along the South Bank between Waterloo and London Bridge (up to Borough Market via Tate Modern). Fantastic views of the city (particularly as it gets dark), and it’s free! Sure it’s touristy but I never tired of it when I lived in London and I still enjoy it now.
Always loads going on and plenty of atmosphere – street entertainers, buskers, skaters underneath the South Bank centre, sand sculptors at OXO tower etc. Plenty of places to stop for a snack / drink (Gabriel’s Wharf etc).
I’d also recommend taking the Thames Clipper service out to the Thames Barrier and back – great views all the way and it really picks up a fair turn of speed once it clears Tower Bridge so quite exhilirating. Stand outside at the stern for the best effect! Check times though, otherwise you could get stuck at Woolwich Arsenal for an hour.
You joked about Soho, but it’s actually cleaned up a lot now and is actually an interesting place to have a wander around.