Why not tell them how inspirational they are to you. To be happily married for 45 years is glorious and lots of people could only dream of that degree of committment.
Take a quiet moment, look inside yourself and think about what your parents (and their 45th anniversary) means to you. That should give you the answer.
Also, opportunities like this give you the chance to say things that you might never say eg if you are a family that doesn't really outwardly demonstrate your love much then writing "I/We love You" is very powerful - and honest.
My parents had their 50th anniversary last year and all 6 of us children took the opportunity to make sure that they both knew just how special they are to us. Both of them are now in poor health and the opportunity to celebrate their lives and their long marriage is now a very important milestone/memory to all of us. I think too many people miss opportunities to say whay they really need or want to say to loved ones.