Mannn! I wish my husband would ask that question!!! He and I have practiced NFP for a while now, but still find it tempting to not abstain when we have talked about abstaining. In regards to it being a mortal sin if you do not manage to abstain, I do not believe it is so. Sure, you and your wife obviously broke your agreement to abstain, but you are married adults, and in the eyes of the church, it is not a sin to have marital relations, even if you were working at abstaining for various reasons. That’s my thought.
As for ideas on things to do with your wife, I would suggest the following:
Private dinner, then dancing afterwards, (kids in bed)
Read the bible together with lit candles (very romantic and non-threatening)
Good relationship-building time where you can go over those books that ask a bazillion and one questions that you adn your wife can then discuss.
Watch a movie with popcorn/chocolate
Write each other poems
Go to karaoke if you like that thing
Take a retreat together
Go to the park
say a prayer over each other before bed, to strengthen and build your relationship during this time of abstinence