“I just want to stay in tonight” is a full explanation. You don’t need a better excuse.
STILLNESS
STILLNESS
Wanting to stay in is already the entire answer. The habit of upgrading it into something more acceptable, tiredness or homework or a headache, slowly teaches you that your own preferences require outside justification, and that is an expensive thing to believe about yourself. People who actually care about you can handle a plain no.
Practise the short version this week: you would rather be home tonight, and that is the end of the sentence. No elaboration, no apology stacked on top of it. Notice that nothing much happens as a result.
Why do you add reasons when a simple no would do?
Free, with nothing to buy inside it.