Seven sisters road isn't that nice, might be better going further down lee valley then across Stamford hill, through to Finsbury Park, the parkland walk to Highgate can be muddy but a nice ride. Looking slightly further north is Alexandra Pallace, bit if a hill with great views over London. ( if not foggy like Sunday ;).
Swaines lanes goes between Highgates old and "new" cemeteries, you pass some interesting gothic buildings .
If you've energy can loop left at top, down Westhill , and do it again ;)