When we went to the princess meal at DL, DD was the only one dressed up. She was slightly over 3.5 at the time. We did ours at dinner on the day that we checked out of our hotel, so I was a bit concerned about what to do for her. We decided to put her princess attire in a bag and put the bag in one of the lockers. When it was close to time to eat, we got the bag, got her changed into all her "bling" and went to the dinner. When she got that outfit on, and had her hair done a little bit, WOW, she really was a princess that night. She had been a bit tired and crabby before that, and afterwards she just glowed. She strutted (yep, not other word for it!) around like she was "all that". She has pictures of her and all the princesses in her princess autograph book, and she sleeps with that dang thing under her pillow! And, when she describes it as she's showing it to people she says "look, I'm a princess, too". To me, that experience was priceless, and as she gets older, I just don't think it will have the same "magic" as that night did.
So, if you take her without the costume, she definitely wouldn't be the only one, and she certainly would still have a great time.
If you want creative ideas on how to make it work, even mid-day, then we'll be glad to help you!

Edited to add, after dinner, after going on the carousel, she wanted that dress off pretty quickly! So the experience was certainly short lived! So much for the "well, she'll probably want to wear the dress all day and all night" thoughts that I had beforehand!
