Pause Startup: this is more of a GBA trick than a Spyro code.
Press + at the Gameboy startup screen and (Nintendo) disappears. It just sits there with Gameboy on the screen untill you hit A,B or the directional pad.

Quick Start:
hold + + after you turn on the game.

All Lands visible in Journal - You can view the page for every Land, even those you have not been to yet.
Enter this code at the 'Press Start' screen.

A whole lotta noise about nothing - you may recognize the code above from Season of Flame. all the codes from Season of Flame will make a noise when entered but they don't seem to do anything.

Why so few codes? - The front end, including the code entry, were reused from Season of Flame. Digital Eclipse intended to put codes in but Vivendi said not to, so they where removed. The one that does work was left in unintentionally.