God is very surprising. (a) Facts about God's nature are surprising. Who would have expected the being who is one and indivisible to also be a Trinity? (b) Facts about what God does are surprising. Who would have expected him to choose insignificant Israel, or... to die for us!?
Surprisingness isn't entailed by incomprehensibility. A text in a language I don't know can be utterly incomprehensible and utterly unsurprising. But on the other hand, insofar as God has surprises for us we do not comprehend him.
Surprisingness implies that not only are there facts about God that we can't figure out, but there are facts about God which our present evidence would lead us to deny--surprising facts. A focus on God's surprisingness may lead to a sceptical thought that we cannot expect to know anything about God. But that's not at all true. For mathematics is continually surprising, and yet is an area where we continue to know more and more. (And this is more than an analogy, since mathematics is a kind of branch of theology, if Augustine is right about mathematical objects existing in the mind of God.)
There some connection between God's holiness and God's surprisingness. God's surprises aren't just like particularly thoughtful birthday presents. They are the surprises of a mysterium tremendum. "Surprise" is thus too weak a word, yet it also correctly suggests a certain whimsy that is a part of God's nature if the whimsical surprises of mathematics and biology are a guide to the mind of God. Humor may be a form of theology, too.