While Steam is always popping up small sales here and there, I felt that it was worth mentioning the current Frontier sale that is on until April 22nd - as two of my favourite games - Planet Coaster and Elite Dangerous, are currently on sale for a massive 65% discount.
Planet Coaster is the effective spiritual successor to the amazing Rollercoaster Tycoon 3, and you can check out one of the parks I built with it here. It's a very intuitive, well built and infinitely customizable Theme Park management simulator, and I can't recommend it highly enough. If you enjoyed the old Rollercoaster Tycoon or Theme Park games, then this is an absolute must for you.
Elite Dangerous is also worth grabbing if you've not had a chance to play it yet. It's a space-flight simulation game that allows you to explore a realistic, actually to scale open world representation of our galaxy, which is frankly an amazing thing when you comprehend the sheer scale of the thing, not to get all Professor Brian Cox on you.
Jurassic World Evolution has a similar discount, and you can get a massive 80% off the LostWinds titles, too.
But seriously, get Planet Coaster. And all the DLC. Really.