Every year I do it a little differently, depending on what day it falls on, how tired I am, how many people show up to help, etc.
Basically, kids drop off their animals and we ID them. This year I got a cool superhero id badge from another librarian that was awesome. At some point while we're closed for the holiday, I come in with whatever help I can collect and my volunteers take pictures. I then upload the pictures into a Souvenir Booklet for each kid. You can see one of this year's booklets here.
At some point, I make some kind of slideshow and/or posts for Facebook. This year I scheduled posts all Friday afternoon and my associate will hopefully turn the full selection of pictures into a slideshow for Utube on Monday. I had 3 friends taking pictures this year and it took about an hour and a half. Loading all the photos into the booklets took about 3 hours because I forgot to tell my helpers to take the photos in groups and not randomly!
This is a great way to promote the library and it offers a fun and fairly labor-free program for kids who don't get to go on vacation over the holiday. It's a nice thing to look forward to and marks the division between the two months of summer.