How to activate the debug toolbar?

It's already built in, as long as you run the development environment.
https://codeigniter.com/user_guide/testi...ug-toolbar
https://codeigniter.com/user_guide/gener...ments.html

Just press the Flame at the lower right corner, and it will pop up.

CI3 had a "similar tool" as well, but not that extensive.
https://codeigniter.com/userguide3/gener...iling.html