Er... that's still not a lot of info!
But just to chuck in some random things (which may not fit your characters' sex, age, profession, or anything else!)
Avoid forests and alleyways (both cliches). Avoid gas stations, car parks and public lavatories for the same reason.
If you want different, there's:
An indoor fish market (yes, they work all night and are very nasty)
A speed-dating venue
An ice rink
A gym
If you want familiar, there's:
The witness's own house or garden
Street or garden where witness has sneaked out for furtive cigarette
A brightly lit all-night supermarket
The 'coats' bedroom at a party (I've always wanted to do a scene where the last person retrieves their coat and there's a body underneath)
If nothing else, these may help you narrow down what you don't want! The best idea will be the one you've come up with yourself, because it's one that gets the juices flowing.