Tower of Druaga and Timeball are pretty old designs. Boxyboy (1MB!) and Drop Off give me that Atari vibe.
EDIT: Don't forget Meteor Blaster!
Wonderboy in Monsterland and World Court Tennis feel like NES games to me.
Well, Monsterland actually is an 80's arcade game and a perfect port aside from the rebranding and sound.
I guess that other Namcot games would count too, like Wonder Momo, Final Lap Twin, Pac Land, Valkyrie Legend, World Circuit and Youkai Douchuki.
Buster Bros and Splash Lake have "simple" timeless gameplay.
I haven't really tried Skweek, but it looks retro enough.
How about Tricky Kick?