It’s veggie burgers for a cause. Collaboration-crazy Shake Shack will serve a special mushroom burger tomorrow at its Upper West Side location, with all proceeds going to Wellness in the Schools. Created with local chef Bill Telepan, the Wellness in the Schools Mushroom Burger ($6.99) is, you guessed it, a mushroom patty topped with pickled shiitake aioli, fried shallots, and Swiss cheese. The week’s great weather is expected to continue tomorrow, so you were probably already thinking of bringing a cooler of beers to Central Park. This doesn’t sound like a bad way to turn it into a picnic.