covid cancelled their wedding, so they gave items to us for next year..how do i thank them?

Just thank them for the gift, I wouldn't mention their canceled wedding. Let them know how thoughtful and kind it was to give you those items.

That's so nice of them. like pp mentioned a nice thank you gift and card will do

"Thank you so much for the generous gift."

That's a bummer that they had to cancel their wedding but that was very nice of them to give you those items!! I think on the thank you card "thank you for your generous gifts and kind heart. We look forward to seeing you at the event. Here's a little something for such a generous gift" Or something like that??

You can definitely just not mention why they had the stuff to give you. If you feel like you want or need to, you can say something like, "I'm sorry your [or cousin's name]'s wedding wasn't able to happen as planned, but I'm so thankful we'll be able to use [whatever they gave you]. It was so nice of you to think of us in this way and we really appreciate the gift. We hope you'll be happy with the way we decide to use [items] and we're really looking forward to celebrate with you."

Thank you all for your helpful comments! I appreciate it!