A lot of things, but most of what they deny is probably worth denying. Where I believe Calvinism denies a truth is that Calvinism by and large denies that God can and very well may use his Foreknowledge (even of what he foresees in a man's life) as a basis for election. The simple fact is that Foreknowledge is stated as a reason for election and the term "foreknowledge" is not theologically defined in the Bible.
Arminianism denies that God has the sovereign right to completely and unilaterally change a mans free will to make him incapable of rejecting the gospel message and incapable of falling away.
IMHO Arminianism puts too much emphasis on free will and Calvinism puts too much emphasis on God's determined will apart from his foreknowledge.
Next question.
I think Calvinism also denies that
GOD BEING GOD
HAS THE CAPACITY
to capsulize a given . . . reality . . . in some dimensions of ‘unknown’ as a Father might play a game or charade with his children.
That God has the capacity to at all times know everything is a given . . .
I think it may be less of a given that God chooses to ‘consciously’ know all things about every thing to the nth degree 24/7 in all context about all components, facets, factors.
How can a truth still be "can and very well may"? Either it's a truth in which a set of propositions prove it or it's just a conjecture thus not proving that Calvinism denies this "truth".