Recently I've playing playing this NES game The Guardian Legend
It's about this big death star thing called the Naju which is full of aliens and hurtling towards earth. You play as the Guardian of Earth who is a chick that can transform into a spaceship.
The Guardian has to destroy the Naju by deactivating safety devices in the "Corridors" that stop the self destruct feature from working. So it's like a Death Star trench run but there's like 12 of them.
In human mode you roam the "Labyrinth" which is the surface of the Naju. It plays like a Zelda 1 esque adventure game, you run around killing big monsters for weapons and upgrades and look for clues on how to get into the Corridors. (Left for you by the now dead original inhabitants of the Naju after getting attacked by aliens)
As a sphess ship you fly through the Corridors in shmup mode and fight whatever giant lobster thing happens to be at the end.
It's cool how it combines two different genres, especially in a NES game. The weapons you find work the same in both play modes which is neato since it kinda blends the line between human and ship.
The japanese boxart is rad as all fuck
http://i.imgur.com/cCKiL5N.jpg