Examples:

• “Best wishes!”
• “Congratulations on your wedding!”
• “Here’s a little something to start your life together.” (if you’re including a gift)
• “Here’s to a long and happy marriage!”
• "I'm seeing a fun-filled life in your future. Congratulations!"
• “I/We wish you guys lots of love now and always.”
• “Here’s to many years of love and happiness!”
• “Best wishes for an amazing future together.”