
I think what was getting me is that the usual "once we get home we will..." techniques weren't working. Neither was dangling the rest of her animal crackers in front of her (she gets one bag a week at the activity I was trying to get home from.)
What I finally did, after 30 minutes of letting her mess around, was force her into the seat. She screamed, I got into the drivers seat, she kept screaming, we drove one and a half blocks forward, she stopped screaming. When we got home she was half asleep and miffed looking, she grumbled at me, snuggled her head down onto my shoulder and nursed to sleep in about 5 minutes once we got inside and into bed.
It was like she was stuck in "on" and I had to trip her circuit breaker to let her turn off.
ETA: not that I'm recommending waiting 30 minutes, it's just that when I tried to force her into the seat after 5 minutes and 15 minutes, I couldn't do it without feeling like I was going to hurt her since she was resisting so strongly.