Shadow Tactics is such a good real-time tactics stealth strategy game that it rejuvenated the genre. There's a chance you already own the sneaky samurai sim, however, and much less of a chance that you own its standalone expansion, Aiko's Choice. It's the latter that is currently being given away for free over on the Epic Games Store.
If that's not enough, wonderful slapstick spaceship builder Kerbal Space Program is free, too.