I think there can be a place for physical stretch goals, but you'd need to be careful with them. I think one place where they can be useful is material extracted from the main product, where minimal development effort is needed for the stretch goal. I'm thinking of things like cards containing various elements on them (conditions, spells/powers, gear), or maybe cardboard pawns using the monster/NPC illustrations you already commissioned. But you should be careful to check the prices for that kind of thing before doing your kickstarter so you know how much they'll set you back and thus how much extra the kickstarter needs to bring in. And you should only do it if the kickstarter is pretty much ready to go (with maybe a brief backer preview/review period), because if the KS is supposed to fund future development (as opposed to recoup the costs for development you already did) there's no guarantee that the price you were quoted for a deck of cards will remain the same a year from now.