Cheaper diapers that aren't as absorbent is a great idea (wish I'd thought of it!). Another thing you might do is start talking about things your daughter will be able to do when she is a big girl, and really play it up.
My second child never wanted to do big girl things until I started talking about, "I can't wait until we get to (insert something FUN).... oh, that's right, you're still a baby. Never mind, we'll have to wait and do that when you're a big girl." From things like who wants to help set the table, to who wants to sit in a booster without straps (sitting at the table!!! NOT in the car, obviously!!!), to who wants to go to a water park..... all things for "big girls," that I knew she would LOVE to do, but gosh, couldn't do while she was still a baby.
I never wanted my girls to grow up too fast though. The "big girl" things I wanted them to do were simple milestone type things like giving up the bottle (at 2 1/2) and potty training (between 2 1/2 and 3), and giving up the binky (around 2 1/2). Even with those milestones accomplished, I made sure they knew that snuggling and cuddling and rocking with Mommy were perfectly acceptable big girl activities and didn't ever have to quit even when they were big (my 6 year old will still rock with me!).
ANYWAY.... that is how I enticed my middle child how embrace the idea of doing big girl things.
As far as daycare and kids her own age, and her saying she's a baby.... have you talked to her daycare teacher about this? Could her teacher be putting that idea in her head as a way to avoid potty training, and other big girl milestones? I knew a few teachers that would loudly complain about their role as teachers, and how these kids were just babies, etc.... it might be something worth looking into.
Her behavior normal to a point.... usually there is someone or something putting the idea in a child's head, that it's better to be a baby. Like if there is a baby at daycare and the teacher is saying, "I can't pay attention to you because I need to deal with the baby...." Or if a child doesn't have to clean up because they are, "just a baby," or if there is a child who gets special meals because they're, "just a baby." Things like that can put ideas in a child's head that being a baby is the better way to go. Again, it's something that might be worth looking into at daycare.
Anyway, hope something here could help!