The registry exists so the couple gets the exact towels they want — and by all means, buy the towels if they're still there. But registries get picked over fast, and more couples every year either skip them or fill them with things nobody enjoys giving. That's where the off-registry gift shines: something with a little more personality than flatware, chosen because you actually know these two people.
The rules of a good off-registry wedding gift: it should be for both of them, it should feel slightly more luxurious than what they'd buy themselves, and it should never create a storage problem in a first apartment. Consumables, experiences, and beautiful useful objects win; novelty "Mr & Mrs" merchandise does not.
Everything below is organized by budget — TOKEN (Under $50), CONSIDERED ($50–$150), and GENEROUS ($150+) — then by the kind of couple you're shopping for.