Yes, you can warm up yesterday's waffles or a batch that you froze for later in the week and they can be just as crispy as freshly made ones. You have to use dry heat, so a microwave won't work. Whether they're from the fridge or freezer, put them straight in a toaster or an oven without thawing them. Use a high temperature if they're from the freezer so that any ice doesn't get a chance to be absorbed into the waffles.