TinkerCAD is free and browser-based. It’s remarkably powerful for something that’s basically explicitly supposed to be simple enough a child could learn to use it.
And I’ll second trying to understand some G-code. When a malfunction causes your print to stop cleanly midway, pretty much your only way to salvage it is to figure out which Z-layer it malfunctioned at, and directly edit the G-code to start where it left off. It’s finicky, but a lifesaver on large prints. Nothing is worse than 30 hour print failing 80% of the way in.