For the groundlings and stinkards, it was mostly oysters, which was what poor people ate back then. Oysters were the popcorn.
Did Shakespeare ever refer to oysters? Of course, including the most famous oyster-expression: "The world's mine oyster." But there are a bunch more:
I did her wrong/Canst tell how an oyster makes his shell?/No/Nor I neither; but I can tell why a snail has a house/Why?/Why, to put ’s head in; not to give it away to his daughters, and leave his horns without a case.You can see they didn't think much of oysters then. Presumably, the oysters/shells were thrown at the actors, and these lines inspired humorous punctuating splats of oyster onto the faces of hams.
I will not be sworn but love may transform me to an oyster; but I’ll take my oath on it, till he have made an oyster of me, he shall never make me such a fool.
Off goes his bonnet to an oyster-wench...
He kiss’d, the last of many doubled kisses/This orient pearl... ‘Good friend,’ quoth he/‘Say, the firm Roman to great Egypt sends/This treasure of an oyster....
He is my father, sir; and, sooth to say/In countenance somewhat doth resemble you/As much as an apple doth an oyster, and all one.
Rich honesty dwells like a miser, sir, in a poor house, as your pearl in your foul oyster.