Hello Jerod!
Welcome to planDisney! I'm always trying to figure out my outfits when visiting
Walt Disney World Resort. I have had to wear sweaters some days and then light dresses other days. It's crazy how it fluctuates sometimes in one day!
I see that you are planning a visit in July which means that you should anticipate some very warm weather. Now, there are some rules about attire in the Parks as Walt Disney World strives to keep guests safe and keep it a family-friendly environment. A shirt or dress with spaghetti straps sounds fine to wear as long as it does not expose
"excessive portions of the skin that may be viewed as inappropriate for a family environment" You will have to make a judgment yourself if your daughter's dress will be inappropriate but also realize that Disney will also be making that judgement as well. As for costumes, anyone over the age of 14 can't wear a full costume unless it is for a special event like
Mickey's Not-So-Scary Halloween Party. You can
"bound" however using regular clothing inspired by certain characters. I'm not that creative to figure out a bounding outfit but I have friends who do a great job picking out colors, accessories, and even Mickey ears to hint at their favorite characters.
Make sure that any clothing you wear has no obscene language or graphics, is not excessively torn, and is not dragging on the ground in a manner that could cause a trip hazard. Don't forget to dress for the weather as it will get hot and it will most likely rain during your trip. I always pack an extra pair of socks and a poncho for my Park days. I hope this helps in your packing!
Happy Planning!
-Nimmy