Fun little secret I found today.
Enter the title screen while holding left+down OR right+down , then select Credits. One of two secret messages will be printed before the thanks.
: For example by exiting the controller screen.
: When the title screen appears you may release the buttons.
March 13 ninja edit:
There's also some apparently unused code related to the NerdTracker II playback code at $8006, but I have no idea what it's supposed to be doing.
These Game Genie Codes will turn Solar Wars into a graphic-less sound test mode:
The A button advances to the next track.