Cake crumbs

Momofuku Milk Bar has a birthday cake recipe that uses toasted cake crumbs in between the layers of their cake. It's something like one layer, some frosting, some cake crumbs, repeat. It was a total pain to make the cake but boy was it delicious. Toasty cake crumbs are amazing.

Use them in an autumn fruit betty, e.g., https://food52.com/recipes/7721-pear-and-quince-brown-butter-betty While the oven is heating, toast/dry out on a baking sheet several cups of cubes. Break some down into crumbs, leave larger pieces for the top. Dial back the sugar if the cake is sweet. Key: Drizzle each layer including the top with about a quarter cup of heavy cream. Heavenly. ;o)

Freeze as suggested.
In addition to the good ideas so far, they could go in a streusel topping, or layered in an ice cream parfait, or even in a tiramisu (depending on flavor preferences).

Freeze them, definitely. With something like cake, don't worry about using it up right away, it freezes beautifully. When you're ready, defrost and sauté them in some butter 'til golden brown. Sprinkle on brown sugar and cinnamon (or other spices) and toss to coat. They're like sweet little croutons and go well over ice cream, pudding, rice pudding, cheesecake, panna cotta, etc.

Also, you can dry the crumbs out and pat them around the sides of an imperfectly iced cake for a lovely look that sceams "I'm elegant", even though it means "this didn't look so pretty" :)

Cake balls (or pops) are super easy. Crumble the cake, add some flavor (ie: fine chopped nuts, dry fruit, etc) add icing flavor of choice (ie: white cake with white icing or yellow cake with pb icing) and mash around til sticky, but not wet. Roll into tight balls and then roll in topping of choice (like crushed pecans or jimmies or something similar) Serve in cupcake papers. I like them chilled. And if you want, add sucker sticks and dip them in chocolate!

How about cake pops? I've never made them but I think baking a cake and tearing it to pieces are the first two steps.

Freeze them and use them on the bottom of the baking dish for any fruit crisp or crumble, or any fruit galette, especially pear, which tends to be quite juicy.
Or use them in a Devonshire pudding! I expect the Devonshire pudding was first created with bits of leftover holiday cake in mind. Recipe here: https://food52.com/recipes/31847-devonshire-pudding (It's also a fine way to get rid of leftover fruitcake, plum pudding, etc.)

Devonshire Pudding is one of those lovely "lost" recipes. ;o)