I've been to the new Tanger outlets in Deer Park. It would be closer for you but they do not have as many stores as their Riverhead counterpart. Check the Tanger website to see if the stores there give you enough variety (I can tell you from my own experience that the Deer Park location only has Chipotle and Johnny Rockets, and they have only one 'home goods' type store; the rest are clothing and accessories.)

There is a multiplex with IMAX at Deer Park.

Just FYI, the date you've chosen is the day before Easter. I'd expect those malls to be VERY crowded, and even more so if the weather is nice.

If you do go out to Riverhead, there are many wineries out there that might be of interest to you.

Both outlets are very safe, as is the drive from LGA.

http://www.tangeroutlet.com/deerpark/directory/