Human Readable GUID

I would use your own encoding scheme. In addition to omitting I and O, for optimal readability it's also a good idea to omit all but one letter out of near-homonym sets (C/E, M/N) and multisyllabic letters, such as W, and of course stick to one case.

As far as length, you could use 60 bits, plus a 4-bit checksum. 64 bits is enough to store the time to millisecond granularity for several thousand years, so it's for all practical purposes unguessable. At say 4 bits per letter, that's 16 letters long. Even half that length is probably plenty.

Another way to think of this is in the form of automobile license plates: 3 letters and 3 numbers is enough to cover a pretty large state, and tends to be very readable. Unless you provide a way for someone to hack codes at high-speed, they certainly won't be guessable at human time scales.

AOL used to use a random combination of two words for the CDs they sent out. You can take the same approach, and just increase the number of words to get the odds that you require.

You could use a Markov Chain trained on English syllables to create a sentence composed of pronounceable-gibberish words. Just add the generated sentence to a database of valid vouchers when you print them (and invalidate them when they're redeemed, of course).