How secure is my iPad on Wi-Fi at an airbnb

Hard to say, as it's impossible to know what the owner may have set up on the wi-fi if anything.

Most AirBnBs use a residencial Wi-Fi so would be as secure as your home Wi-Fi for most regular usage.

Do note there may be specific restrictions in place, and assume the AirBnB opener can verify your navigation with their ISP if they want to, so don't access websites you wouldn't want anyone to know about.

For normal things like website account access and navigation it should be fine.

You should assume the worst and follow the same precautions you would on a public wifi.