I've had pretty good experiences with handing them Linux machines (usually openSUSE + Xfce). There's an initial adjustment period as they figure out "okay the orange fox replaces the blue e", but quite a few people I know who made that switch are now at least reasonably productive. Chromebooks are another option here that I've seen be pretty successful, and they have the advantage of being absurdly cheap.

The absolute biggest bang for the buck: ad-blockers. Fewer sketchy links to click on, and absolutely nothing to relearn.