A quick and easy one and common-themed one here is Drop Zone, based on a certain "Apple classic", which I assume is Sabotage (also cloned as Paratroopers as the hidden game on Gravitrex and Commando Raid on the 2600 is similar, although with the added feature of buildings that you have to protect), as you control a cannon at the bottom center of the screen and shoot down paratroopers before they land. You get five lives, since if one of them lands they'll plant a stick of dynamite next to you and it's game over if that happens five times.
I like the screams of when you kill a paratrooper, along with the bullets hitting metal of the planes (which take several before they're destroyed) and the graphic of the planes going down in black smoke. Nice touch with the clock that appears occasionally that you can shoot to slow down the action for a few seconds.
Good luck making 300/having a game last for even five minutes though! It'd be nice if there was an additional power-up to gave an extra life or destroying all onscreen paratroopers or something though.