I just got the voucher today, and with the Steam Summer Sale on, I need suggestions. I already have 80something games so you might want to check my library (Adameh on Steam)

Only 80? You've got plenty more to buy! :p

I recommend Starbound, Shadowrun Returns, Edge of Space, Deux EX: HR (and all the older ones), Space Engineers, Dishonored, GTA Complete, Tomb Raider, any of the F.E.A.R. games, and there are plenty more! Hard Reset is really good, I just recently started playing it, though I've had it in my library for ages. Saints Row IV is good, as is Saints Row The Third, Civilization V, etc etc. I could write for days.

I just bought Age of Empires 2 HD for pretty cheap. I've been looking for that game for ages.

Europe Universalis IV

You should send me a PM with the voucher code. I don't have 80 games like you yet.

Sonic 4 episode 2

Already have it

Ys Origin.

Dont Starve

Never Spend it, always tell your self you're going to buy something but you're waiting for the right time.