Rubble piles, parks, setting buildings back from the road with plazas in front e.g. that might have been markets or out door cafes or similar in happier times, 'curved' or bendy streets that don't run at 90* angles in a grid are good too - might be worth looking at Google Maps at some older 'classic' European cities for inspiration.
Euro cities have tended to grow organically with bits sprouting and being redeveloped and changed and repurposed seemingly at whim
If you're doing it in detail then other things like public toilets, metro/above ground subway entrances/bus stops, info kiosks, street stalls, clusters of bins, fountains/statues/benches and similar can be used to break up even long lines of sight.
I'd also make as many buildings as possible with accessible insides - just something simple like a lift off roof. Even if players don't use them all the time you can guarantee some smart arse will want to use the ones you make solid
Sounds really promising anyway - look forward to some piccies when done