What is the best?
My boyfriend and I did this a lot in the initial stages of our relationship. We did it even for valentine's day. Just form a code of your own, encrypt your data (Proposal, in this case) and give it to her. Let her brainstorm till she doesn't get it. I personally like creating codes of my own. But you can do Morse code, which is pretty common. You can even use your qwerty keyboard. For every letter on your qwerty keypad, there is a symbol or some other character that corresponds to it if you press on the button. So maybe in place of H use * sign as I see on my keypad. There are easy ones where you just use letters. Maybe go 2 letters up. So if your original message contains a letter B then in the encrypted form it will be D And so on. This one was pretty easy. You can make it tough by maybe recurring addition or multiplication or even division. Be as creative as you can be. Have fun. Hope it helps.