I'm really surprised Krita wasn't mentioned already. It's open source, cross platform and actively developed. It's interface is not as simple as Paint's interface but it's also very intuitive, I think. It is mainly used for sketching, drawing and painting and has a lot of natural brushes, but it can also do simple stuff like cropping or resizing.
If you start Krita cropping can be done by opening your file through dragging or through the dialog. Then you press C or click the crop tool in the toolbar and select the area. Then press enter or make a double click to confirm.
It has also support for transparency, layers, filters, a lot of file formats (including raw) and is a real alternative to photoshop (with maybe some of the newer features missing).
I don't know if I can convince a ms paint enthusiast with that but why bother about the support for paint anyway. Is it really updated that much. For all i know it's just an exe in the system folder which can be backed up to any location, even a usb, but I might be wrong about that.