Here's an idea:

Establish a small secret value X (for example: X = 5).

Establish a rule for every character of your string.

Assign the characters numeric values (for example ASCII code).

```
X = 5
Position: [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ 8 ] [ 9 ]
Rule: +2x +x -x -2x +2x -x -x +2x -2x +x
```

Example for the string:

**thisiscool**
```
Position: [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ 8 ] [ 9 ]
-------------------------------------------------------------------------
Char: [ t ] [ h ] [ i ] [ s ] [ i ] [ s ] [ c ] [ o ] [ o ] [ l ]
ASCII: [116] [104] [105] [115] [105] [115] [099] [111] [111] [108]
-------------------------------------------------------------------------
pos(0) = 116 + 2x = 126
pos(1) = 104 + x = 109
pos(2) = 105 - x = 100
pos(3) = 115 - 2x = 105
pos(4) = 105 + 2x = 115
pos(5) = 115 - x = 110
pos(6) = 099 - x = 94
pos(7) = 111 + 2x = 121
pos(8) = 111 - 2x = 101
pos(9) = 108 + x = 113
```

Recoded back to ASCII:

**~mdisn^yeq**

You may now handwrite this new string in the back of your business card.

To decrypt you'll need to reverse the process and perform the inverse rules for every position.

Useful link:

https://www.branah.com/ascii-converter
**EDIT:**
Using this example as a starting point, the possibilities are endless. You can have 7 different sets of rules, one for each day of the week and your value X can represent the day of the month. The same initial string would always be different if not encoded on the same day.

Use your imagination.