"Thank you, have fun."
Well, I would say you often use תּוֹדָה לְךָ in combination with a name: תּוֹדָה לְךָ דָּוִד עַל עוּגִיּוֹת הַשּׁוֹקוֹלָד thank you, David, for this chocolatate cookies. Or if you want to stress someones merits: סוּפֶּרְמֶן, תּוֹדָה לְךָ הַמְּדִינָה הַזֹּאת שׁוּב בְּטוּחָה Superman, thanks to you the country is safe again. But as the simple reaction to a kindness, תּוֹדָה or תּוֹדָה רַבָּה is much more common and idiomatic, although תּוֹדָה לְךָ is still possible.
I had used תהנו, which it gave in the list of possible answers, and it told me I had a typo, showing the correct answer as תהנה; it was definitely NOT just giving alternative answers. I'm at a loss as when to use which form of this word! Is this one of those things where actual usage differs from the official rules?
This is imperative, which has three forms - תהנה (used when addressing a man), תהני (used when addressing a woman) and תהנו (used when addressing multiple people) - and this is listed as the default answer. Saying "Have fun" specifies neither gender, nor number, so all three should be correct. If it corrected you, it means either 1) you made another typo that you didn't notice or 2) it's a bug. Either way, nothing you can do about it. It happens to everybody.