A very fun idea, and a couple of suggestions (from my perspective as a longtime childcare provider) :
I would not equate going to the potty with 'being a big girl'. The reason being, if she has some regression after baby comes (which is very, very common in this situation), this can conflict with her 'big girl' status to herself. If Big Girls go potty, and she decides she needs to be back in diapers ( it does happen), we don't want her to think she's a Little Girl or a Baby again. So, perhaps "You Did It!" could be a nice message on a cake. No Big Girl talk, if you can help yourself.
Otherwise, keep it low-key. Nice gift-- how about some cute training unders from Hanna Andersenn or another quality brand? Very in keeping with her successful progress.
Please take a look at this site on toilet learning (below) and remember, too, that once the incentives of potty parties, etc, disappear, she might be less interested. Try to take this in stride. Changes often trigger regression, so keep cool and be prepared.
http://www.betterkidcare.psu.edu/AngelUnits/OneHour/Toile...
Have fun.:)