Yeah - the upper island is weird. I usually go up the east side to that bridge over the FDR around 119thish, go across to Adam C Powell up to 155th street (there's a bridge there - go against traffic the last block on the sidewalk, cross over with the light, hang a left and start climbing that hill. It's separated from the road with a fence). At almost the top of that hill is a traffic light - to your right is the path again. Dirty with crap around (keep your eyes open for the tiny green sign that tells you where to cross at the bottom of the hill) but then it's a nice ride past Yankee Stadium for a few miles along Harlem River Drive. The thing that worries me around there is that there's no way out if something happens along the way (and an easy spot for an ambush) so I usually time it so I'm not up there past sundown.

Rough map: http://www.gmap-pedometer.com/?r=395586

The first time I went I rode up St. Nick Ave. Completely wiped after climbing for what seemed like 30 blocks and got lost when it curved and started coming back down in the 180's. Wound up carrying my bike down some steps (looking at the map that turned into Fort George Ave and 181st) and there's (what seems like) a forgotten path on the other side of the Harlem River Drive through the woods. Pretty cool - but also very deserted and evidence of a lot of homeless and drug use there. Most of the entrances I past were chained and locked up but ends at the same point as the other path where it crosses over.

Another time I went I didn't think I could ride near that bridge on 155th, went across 151st and wound up against a wall of rock and under 155th - carryed my bike up 100+ steps. So far the link I posted has been the easier and fastest way around the top without doing that little triangle at the very top.

If anyone wants to post their route go for it - I'm curious what others do.