Personally I'm a cat person, but you can't dress them up without risking twelve stiches worth of feline fury.

BARC Shelter presents:

19th Annual Dog Parade: Ahoy Mateys! Dogs of the Seven Seas

Meet at BARC Shelter at 11:30a with your costumed dog to line up for parade through the streets of Williamsburg. Show begins at 1:30 at McCarren Park with prizes and performances by Jollyship the Whiz-Bang and the Hungry March Band.

from Nonsense NYC

Robert Smithson's Floating Island is set to be lauched from Pier 46 (@ charles street) this Saturday at an opening party from 5-7. The island will circle Manhattan for 10 days.

Details here

An approximate schedule (due to weather and tides):
Schedule (PDF)

Personally I'll be out at Grand Ferry Park in Williamsburg (L to Bedford, south to Grand, right on grand, walk downhill to the water) which has a view of the old Domino Sugar factory and the Williamsburg bridge.

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To make a day out of it, stop by the High Line Block Party thrown by The Kitchen which features a diverse number of performances and booths featuring local food and art activities. From the press release, this looks to be a family friendly event for those with little ones.