Well the official dress code at the signatures is pretty loose. No tank tops on men, no frayed clothing, no hats, no bathing suits. Hemmed shorts are okay, jeans are okay, tennis shoes or sandals are okay. Probably a little more than half of the people "dress" for dinner but the rest will be dressed fairly casually. If that's the only thing holding you back from booking a signature, I wouldn't let it stop you.
So for most romantic where I think the atmosphere is casual enough that park attire would not stand out, I would suggest Narcoosees at the GF or Artist Point at WL. Both are convenient to MK.