many cards include effects whose cost requires you to return a DON
Adelbert Steiner Aerith Gainsborough Ashe, Princess of Dalmasca Beatrix, Loyal General Cecil, Redeemed Paladin Cid, Timeless Artificer Cloud, Midgar Mercenary Cloud, Planet's Champion Dion, Bahamut's Dominant Garnet, Princess of Alexandria Hope Estheim Joshua, Phoenix's Dominant Judge Magister Gabranth Lightning, Army of One Minwu, White Mage Noctis, Prince of Lucis Rinoa Heartilly Rosa, Resolute White Mage Rufus Shinra Serah Farron Snow Villiers Squall, SeeD Mercenary Tidus, Blitzball Star Weapons Vendor Yuna, Hope of Spira Zack Fair Zidane, Tantalus Thief
With that said, Final Showdowns first effect removes Resilient Roadrunners Haste ability as well
Knowledge is the greatest tool of the mortal mind, outweighing anything made by mortal hands
The most obvious target would be extra combat spells, especially if you've adding on some mana generators from attacking (various treasure generators, Neheb, some dragons)