HECK - I am not even married and EVEN I KNOW THE ANSWER TO THIS ONE!
Stick to your plans and say...
"I have something already planned, but these free tickets came up....what would you like to do??"
Then if she says yes...fill the Hotel room with roses and take her for a romantic dinner after the game, etc..etc..etc..
If she says no...put the tickets on Craiglist and sell em to pay for your trip to the DesMoines. Wife = 1st, Football = 2nd (sadly)