Two of the most successful gifts I've ever given are T-shirts I had made for two recipients who are very close to my heart. One is my husband, and the other is one of my best friends. (Admittedly, they both wear theirs exclusively for exercising, but that's fine with me. They wear them.)
When I started dating my German husband, I thought it was hilarious how Herr Doktor was used 'for real'. He would get mail addressed to Herr Doktor. He would be introduced by one German to another as Herr Doktor. They all took it very seriously, and they all played by the rules: Another Doktor could call simply call him Herr, but any doctorate-free person would have to call him Herr Doktor. (And if he was also a professor, he'd be Herr Professor Doktor. And if he had two doctorates, he'd be Herr Professor Doktor Doktor. For real!)
Anyway, it was an easy jump to start calling him Mister Doctor in respectful jest. And then adding MD those his initials. Suddenly, he had a monogram that was our inside joke. I bought him Scrabble piece cufflinks (M and D), I had an early gift of an iPod inscribed with his initials, I had made for him a dark grey T-shirt with the four letters in lowercase. It was cute, he loved opening it, and he still wears it.
For my best friend, I gave her a T-shirt that simply says Hh.
One long-ago, blurry evening (most likely tipsy and having a heart-to-heart about how awesome our friendship was), I said something about being good enough friends that I could put my head on her shoulder. The next day she signed off an email with hh, which stood for 'head on shoulder' and we've never stopped using it. Once texts replaced emails, the first H got capitalised, and it became Hh. (That she got married at the Hotel Healdsburg in Sonoma County, where the logo is H H, reinforced the awesomeness of it all.)
In either case, an outside person looking at one of these shirts wouldn't know where to begin. The message is cryptic enough to be cool, and the letters are minimalist to look stylish. These traits are key in making a gift like this work. If there's too much information, your recipient will be embarrassed to wear it in public. And you'll be embarrassed you gave such a gift.
So, where do you get started? At a site like this, which makes customisable T-shirts (and also has a fun and stylish shirt collection for men if you need some inspiration).
This could be the best 20-quid gift you ever give!