Memphis Kiddie Park in Brooklyn is offering a mid-week special where you get an all-you-can-ride wristband for $8.50.
Single rides are usually $1.40. You can get the cost down to 90 cents or so by buying tickets in bulk, but this is a steal. My 4 year old went on 36 rides in just under 2 hours today (yes, I counted... I'm on FW aren't I?). So his per ride cost was under a quarter.
Lines are practically non-existent; he only had to wait for a ride twice all day.
The special runs for the entire month of September on Mondays through Thursdays, from 10am to 3pm. No coupon required; it's plastered all over the park.
Go here to see the line up of rides (11 in total).

