Ask them if you can help with the decorations because you LOVE to do this type of thing, but be careful not to make it sound like you're doing it because you think they'll screw it up! Chances are they'd be happy to have you help with that while they're busy preparing food and inviting friends and family. I love this type of thing too and I helped with mine and am so glad I did because it was a fun way for me to be involved. Plus it's great practice for the stress of decorating for the wedding!