If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, those must be paid to cast the spell.

If the spell you cast has {X} in its mana cost, you must choose 0 as the value of X.

A permanent spell is an artifact spell, a creature spell, an enchantment spell, or a planeswalker spell.

Consider the other permanents you control as the triggered ability resolves to determine which spell you can cast. If the only other permanents you control are lands, or in the unlikely event you don't control any other permanents, you may cast a permanent spell with mana value 1 from your hand.

If a permanent has {X} in its mana cost, X is considered to be 0.