We've drawn pictures of things we have seen
That's the thing eh? Children go through cycles of how they want to learn about the world around them, and one adapts to that. :) It's all good.
There are some useful sources out there for making or using nature journals. The following are pages I have used.
The nature study pages from Patriots of the Republic.
Nadene of Practical pages had a nature study journal that we used off and on as well.
NotebookingPages.com has a lovely nature journal. I get it with my membership which is totally worth it by the way. :)
When you journal you can journal all sorts of things and even create graphs to go with them. You can go out one day and count how many buggy type critters you can find and document that one a graph. You can do this flower/flower colours, types of trees, birds, animals etc that you see.
So much can be learned from studying the nature in the world around you and taking the time to document what you learn. :)
Have fun with it!