I have used https://www.diagrams.net/index.html

for all sorts of planning /designing from apps, to animation  and websites.

It has a large range of categories. The ones that I have used with my students were flow charts for their basic algorithms such as calculating a parking ticket in the city centre, mind mapping an animation project about a social issue, wireframes for designing a website for AIT classes, site  maps and etc. All free and easy to use. Depending  on the class’s project requirement , it was quite easy to manipulate and use the mapping resources here.