I started with a simple on screen 'watch variable', so I can visually inspect values over time. I added simple click drag edit support so I can then edit a value too. Over time I've added a (very simple) console, a simple immediate mode debug UI, graphing for the watch variables, and a little track editor (with the debug UI). Again the key is to build only what you need and only when you need it. It sounds quite a lot when I write it down, but it's all super simple and only supports the barest features that I actually need.