dont walk in front of me when im late for work.
anyway. if you're here during the weekend i recommend governor's island. the ferry is free. i was just there last weekend (i've been living here in nyc for 20 years and i've hardly ever been to touristy places). it's pretty cool and less crazy than ellis island (you do get a view of downtown manhattan, the brooklyn crossings, jersey city and the view of ellis island + lady liberty)
http://www.govisland.com/
it's a short 10 minute ride from the tip of lower manhattan / battery park's gov island ferry (right next to south ferry to staten island), so you can try fit it into your downtown touring schedule. friday bike rental is free for an hour, but i recommend you leave a few hours before the last ferry out or else there would be a long line departing the island.
another tip: try to plan your day ahead and buy tickets online whenever possible. pick them up at will call. it saves you tons of time and you'll thank me when you don't have to stand in line for an hour in 90 degree weather (although this week its 60s to 80s)