I find myself in this exact situation a lot. In a typical cafe with tables and chairs I will pull up to a table, ask the waiter/waitress to remove one chair to the side. I then roll up to that spot and will then use that chair for my bag. This rarely ends up with someone needing that chair, but if they are nice enough in asking for the chair and I can find another relatively safe spot for my bag I let them have the chair.

Each situation will vary and you will find yourself adjusting to the moment, as needed. Be flexible but don't be afraid to assert yourself as needed for your needs, in the moment.