Term for something you don't know you'll like until you discover it?

In general, serendipity is the act of finding something valuable or delightful when you are not looking for it.

I'm not aware of a related concrete / more concrete noun for the object found or the idea the person has.

There are also the fairly common phrases pleasant surprise and pleasantly surprised. (But not as succinct as Edwin Ashworth's serendipity)

If it is something which one learns to appreciate, 'an acquired taste' fits the bill.

'I didn't know anything about India Pale Ale until I acquired a taste for it.' When my wife persuaded me to listen to The Archers, I found that I acquired a taste for it every day'.