Check out the book “Miss Manners Guide to a Dignified Wedding.”. Great book and great comcept. It will tell you how to be excruciatingly correct in every situation.
Try something like this.
Dear xxx
We were so happy that you could join us for our wedding.
Thank you so much for the (insert name of gift here). (say a few nice words about the gidt here, favor adjectives like generous, beautiful, perfect and usefull)
Thanks again for making our day so (special,meaningful,womderful,memorable). (insert bracha here)
With love,
The newly married couple