Might take some ingenious control mapping, but the 1 player Karate Champ is a fun vertical game that'll run on a 266 no problem. I play it with one joy and 4 butons fairly naturally now.
Are you going to have a spinner or trackball? Kids like
Bomb Bee and
Cutie-Q. Both verts, both crosses between pinball and breakout, both designed by the guy who designed PacMan.
Speaking of whom: Jr. Pacman and Pacman Plus (both verts).
Capcom Bowling (trackball, not particularly classic, vert)
Gorf! (vert)
Bosconian (?)
Racer X (?)
Joust II is vertical with the same basic gameplay as the original.
Kick(man) (?)
That's all i can think of right now...