For hard candy you really want to look for stuff that is solid all the way through rather than having a hard candy shell. For example Tootsie Roll Pops and Blow Pops are hard to reuse because the tootsie roll and gum and Tootsie Rolls belong in the chewy category. It's not impossible to reuse these, just more difficult. Now, the best way to reuse hard candy in most cases is to separate it by flavor and then crush it into powder. Once powdered you have several options:
The candy can be mixed into melted chocolate. Mint in dark chocolate. Butterscotch in milk chocolate.
It can be mixed into a hot drink such as coffee, tea or hot chocolate. Cold drinks like milkshakes or smoothies also work well. Cinnamon in hot chocolate. Peppermint in a milkshake
Cool/room temperature drinks require that hard candy be crushed into an extremely fine powder otherwise it won't dissolve. Cherry jolly rancher mixed into a Dr. Pepper Butterscotch mixed into Mountain Dew. (I'm not saying I recommend or that I've ever tried it just know that you have options)
Or mixed into Icing for cakes or cupcakes Self-explanatory