" I think that it would be better to decide together and agree on a place, but this also seems less confident "
This does not seem less confident. It plain old IS. By 'deciding together', you are removing at least one of your testicles (50%) from the relationship. I would even go as far as to say that you should'nt ask if she is a vegan or not, although you could scope out several places that can cater for this genetic deficiancy - should the need arise. If she is, you can just tell her that you "assumed she was normal". Then make sure you dont do it again.
Anyway, i think that sorting out this kinda thing before the event removes the valuable air of mystery. You could arrange to do ONE thing with her, then , when she turns up, do something completly different.