Buy your partner a real greeting card, write down your feelings and drop it in the mail. When it arrives, they will appreciate the time you took to shop for a card, write down your feelings and mail it. It will be a keepsake and really stand out from the modern day texts and emails that we've become so used to.
Make a list of the reasons that you love your partner. Have it printed nicely, frame it and surprise them. Make it specific to your love. They will know you put a lot of thought into it and they will cherish it forever. It will be a reminder to you both about the reasons you fell in love.
Create a photo album of your relationship. Start with the earliest photos and finish with the most recent ones. Be sure to leave plenty of room at the end for future photos and surprise your mate with the album. You may even set up a photo shoot to have professional portrait made to add to the album. Look through it together often to remember the earliest days of your relationship and to dream about the future.
This one will take some research or the help of a friend, but will be well worth the results. When you are looking for an out of this world romantic way to tell your lover just how you feel, write down exactly what is in your heart and have it translated into a language that you're certain they don't understand. Do not tell them what language it is, just that once they translate the letter they will have a big surprise.