Ask HN: Is there a good alternative to software registration keys?

I'm about to introduce a new revenue model to a service I run with no user accounts. Users will purchase short-term vouchers to enable certain features in the app, and I'm planning to generate random 32-character strings that users can paste into the app once they've purchased.

But software keys are unwieldy, not nice to look at, and often users get confused (I've used keys like this in the past and got a lot of support requests).

Anyone got a better approach?

I've considered using images with data embedded and hashing them, using a bitcoin-style seed word approach, but nothing really feels right.