Bring along everything you need for the day in this New York Mets Big Logo Bungee backpack from FOCO. It features multiple types of pockets to keep all of your stuff organized and easily accessible. Additionally, the adjustable straps and light padding make carrying around this New York Mets backpack more comfortable. $46 on Lids.com.
Make sure you buy your backpack that you can not bring into Citi Field.