Part of the challenge here is that you'd like to say to people, "people like you tried and liked X activity." So you'll need to collect a lot of data on what people tried and liked.
More specifically, you probably need people to enter that data for you. So you might want to combine this with some other business/site/something. For instance, travel ideas would go well with a travel web site, Amazon already does a bit of this for books and so on.
Though that's just basic recommendations. Part of the problem is that "out of their comfort zone" implies a significant likelihood of not liking the activity or object. Part of the problem is distinguishing risky attempts ("I didn't expect to like this, but it was awesome!") from plain old bad ideas ("yeah, this sucks. I kind of expected it to.")
It'd probably be hard to get people to keep coming back to an "expand your horizons" web site that did nothing but collect data and recommend stepping outside their comfort zone -- most people don't want to spend all their time outside their comfort zone, and most people only do a few things that are particularly hard for them, so they don't have a lot of data to give you offhand.
It seems like there must be somewhere that people are already collecting this data, I just don't know where it is, or what gimmick you could use to get it from people. I think just asking them will give you pretty noisy results, though.