The correct answers are I'd love to, but I already have plans this weekend. Have fun, though!, Thank you so much for the invite, but I won't be able to make it this time., and Sounds like a blast, but I'm afraid I'm swamped with chores this weekend.

When saying no politely to a social invitation, it is best practice to express appreciation for the invite, state that you cannot attend, and offer a brief, gentle excuse. Being too blunt can come across as rude or aggressive in English!