You do not need a dramatic reason to prefer a quieter evening.

People invent obligations because a preference seems too thin to offer. A headache, a delivery, an early start, anything that sounds like a reason, because wanting to be at home with the lamp on does not feel like enough to say out loud. It is enough. The wish for a quiet evening is a fact about you rather than a claim that needs supporting evidence.

Decline the next thing plainly and warmly, without building a case. Something is not for me tonight, said kindly, works better than a story you will have to remember. Offer another time if you want one, and let the quiet evening be the whole reason.