What is your budget? You mention a couple things from Mickey Ears (inexpensive) to trips to Disney (expensive) so it's difficult to give great ideas without a budget in mind.....
That being said....1st years in our house were special too
and we wanted something that would be around forever vs. a toy (not that we didn't do that in addition
)
1st Christmas was a rocking chair
1st birthday was a cedar chest for our daughter/a trunk for our son
Pandora charm bracelets are HUGE right now and relatively price friendly. You could purchase a bracelet with a specific bead that means something to you ~ cake for birthday, birthstone, a girl, there are so many to choose from and each special occasion can be celebrated with an additional charm ~ Easter Egg, bunny, Christmas tree, soccer balls, footballs, etc. depending on what she is into at that stage. It's something that will "grow" with her.
Add a pearl necklaces are great but what I would recommend is picking up a full strand of pearls at your jewel of choice and have them disassemble the strand so you can give them to her one by one to make the strand. My reason for suggesting this method vs. buying them individually or by the inch (usually 4 pearls) is because a nice strand of pearls already matched quality wise will cost you less in the long run. Single pearls run approx $35 per pearl while the 4 pearl cards (by the inch) run about $100 depending on the size pearl. A standard 18" strand has approx 50 pearls on it so you do the math. If you took an add a pearl necklace to a jewler to be appraised it will not appraise for the amount you paid out because that cost per pearl factors in labor & materials to string the pearls, not just the cost of the single pearl.
Good luck in whatever you choose ~ you AND your niece are lucky to have each other