Let Grandma spend time spoiling baby, let baby have a chance to be doted on my Grandma, let your darling husband spoil you with a once in a lifetime vacation, and enjoy every minute of it! What an opportunity for all of you! Don't feel guilty about it and don't pass it up.
You and your husband were together before the baby came along, and after the baby's all grown and gone it will be just the two of you again. You two need each other so you can be the best parents for your baby.
Have a wonderful time, and make sure to share some pictures with us here after you return! I can pretty much guarantee that is a trip I'll never have the chance to take, but I'd sure enjoy seeing pictures!