@Angle
GIMP is full of stuff, but it's possible to use it like MS Paint if you want to, and it's mature. So, I recommend it.
Unfortunately, 'draw a circle' / 'draw a square' is really 'select a circle/square, fill it, shrink selection by desired line width, then clear it, ideally on a new transparent layer'. Which isn't horribly awkward once you get used to it, but is still off-putting to new users.
@enkiv2 I actually found a slightly less fiddly way to do it - select with box, use selection -> border, desired width, then fill with foreground color. Still a pain, but the tools give you enough other flexibility to be worth it in my opinion. XD
@enkiv2 @Angle One of these days I want to make a vector drawing program that…isn't aimed toward the way artists think. I know that seems weird, but I often find myself needing to make diagrams, technical drawings, annotate or clean up graphs and other plots, and something like a graphical command line with constraints would be wonderful.