My sugar cookies taste dry/stale
Staleness in baked goods is caused by starch retrogradation. Basically as your baked goods age, the structure of the starches goes from fluffy to a more crystalline form, and this forces some of the water out of them. Unfortunately, once this has happened, there's really no way to get the water back in, as the stale flavor and feeling are more related to the starches crystallizing than to the loss of water.

Sometimes warming the starches will temporarily soften them again, but this will be temporary, and when they cool they'll be even more stale than before. You could try gently warming one cookie to see if it helps, and then warming the rest right before serving. If the cookies are already decorated, it may complicate this enough that it would be easier to just make a new batch.

As a last ditch, you could try adding a bit of simple syrup to the undersides of the cookies and letting it soak in. They won't taste "fresh" again, but it will make them less dry.