Volcanoes!
There are a few ways you can make your own volcano - you can even make them erupt! The easiest way is to use a cone of paper or card - here's a template that has a cutaway of inside the volcano.
The classic choice is to make it with papier macher;
See this think for more info;
https://www.learning4kids.net/2012/04/11/how-to-make-a-homemade-volcano/
You can also use card to make an erupting volcano;
https://www.nhm.ac.uk/discover/how-to-make-a-volcano.html
Or even sand, clay or play doh!!
https://www.bbc.co.uk/cbeebies/makes/nina-and-the-neurons-volcano
If you really want to challenge yourself, you can make a complete replica of Mount Fuji in Japan that can be split in half by following this template here - warning, you'll have to be really good at making models!
You could even make a Lego volcano!
https://www.roevalleyarts.com/stay-at-home-art-club/for-kids/day-1-build-your-own-lego-volcano
Or even Minecraft!
https://www.wikihow.com/Create-a-Volcano-in-Minecraft