Frisky Tom (Game Boy)
Stop those pesky mice from festering and destroying your irrigation system!
A somewhat functioning debug menu exists at 0:3C00. The developers disabled it by placing a ret at the very beginning instead of loading the keycode register value stored at 0:C007. Use the following Game Genie codes C0C-02C-E6A + 07C-01C-E6A + FAC-00C-C4D to re-enable the menu.
You have to press Start and Select once Frisky walks onto a stage. While holding Start and Select, press Up and Down in succession and then the menu should load. Notice that you cannot enter the menu while playing, because Start triggers the pause screen and because of the game's coding quality, the pause screen is a simple polling loop (for releasing the Start button), which stalls the mainloop, which would call the menu.
The menu was coded for 7 options, but only five of them go used. Press Up/Down to move the cursor, press Left/Right to change the value. Press Start to exit the menu. Changing the values of unused options will clobber some (scratchpad?) memory, which might be undesirable. While the values are written back to the proper RAM locations, nothing obvious changes after editing the values, so the menu might not work as intended anymore.