You need an emergency lock out key for Her Van. I have spare keys for both our keyed cars and the house tucked in my wallet. They are actually the Real Metal things, but the idea is the same.
Luckily, the Prius doesn't let you lock the car if the keys still inside. So I haven't had the problem for two years now.
Mind you, I USED to have the that problem all the time with the keyed Escort. I considered the feature in the Prius to be a definitely advantage to old forgetful here.