If you get the Passport at Longleat, it works out cheaper. But you need more than one day to do everything. The good thing about the passport is that you can come back at a later date and do the things you didn't have time to do. It's valid for up to 6 months I think - could even be a year.
The safari park I found a bit disappointing. I've been to Knowsley and thought that was better. People do still stop in the overtaking lanes which is really annoying and it can get horrendously busy.
The rest of the park is good though. They have a bat cave which was my favourite by far - actual bats flying over your head, so close they brush your hair!
They've a small animal zoo for the kiddies, a butterfly house, a maze, a train ride, a boat ride (the only way you get to see the gorilla), a house tour, parrot show, kiddies adventure playground (huge and great fun!) etc etc. All of this is included in the price of the passport. My kids loved it and want to go back.
If you are planning on going, take a picnic because the food is dire and expensive. And don't think you'll get it done in one day, you simply won't.