One possibility would be the park area right up at the top of the Peak (go to the Peak Tram station, but take the road that keeps going up the hill to the proper peak (near the radio masts). I think you could get a taxi all the way up there on a weekday (the road is closed on Sundays I think).
Alternatively there's a picnic spot with nice views at the end of High West, but that involves a 20 min walk from the nearest point you could get a taxi (which would be on Luggard Road going round the Peak).
There's also a picnic area about half way along Black's Link between Wanchai Gap and Wong Nai Chung gap which has good views over the South Side. You'd need to get a taxi to one end or the other and walk for about 15 minutes.
These would almost certainly all be deserted after dark so you'd be free to get up to whatever you want
