I could have written a similar post except DD2 birthday is in just over a week and I am struggling for ideas for that let alone Christmas.

So far I have gotten her some Giggle & Hoot PJ's and a beachbag filled with bucket, ball, frisbee and sand shapers. Now I just have to wait and see what she gets for her birthday.

The things I am thinking are things DD1 does not like sharing. Fold out lounge, trike so they can both ride, fold out camp chair, pram/stroller.
I actually decided to get rid of a few of DD1's toys so that DD2 can get some new stuff for herself because I did not like the whole thought of everything for her is handed down.

Good luck, I will also be interested in others ideas