Just ask her lots of questions for fun (you can find lots of lists of these around, used by teachers etc) "if animals could talk what would they say?" "What would you do if you could fly?" "If you could invent something to help your family, what would it be?"

You can also do this while watching TV/movies/playing games together, try and think of questions about the characters or world you're watching, what and why they're doing what they're doing. Even if they're very goofy questions, it's still good for getting her to engage with whatever she's watching.

Also read lots with her, libraries might even have sections set aside for kids books that deal with critical thinking or asking questions.