Here's a collection of minor tweaks you can do in the Super Mario 64 decompilation project.

To skip the intro peach cutscene when you start a new file, in init_level in level_update.c, change:

if (gCurrDemoInput != NULL) {
    set_mario_action(gMarioState, ACT_IDLE, 0);
} else if (gDebugLevelSelect == 0) {
    if (gMarioState->action != ACT_UNINITIALIZED) {
        if (save_file_exists(gCurrSaveFileNum - 1)) {
            set_mario_action(gMarioState, ACT_IDLE, 0);
        } else {
            set_mario_action(gMarioState, ACT_INTRO_CUTSCENE, 0);
            val4 = 1;
        }
    }
}

to

if (gCurrDemoInput != NULL) {
    set_mario_action(gMarioState, ACT_IDLE, 0);
} else if (gDebugLevelSelect == 0) {
    if (gMarioState->action != ACT_UNINITIALIZED) {
            set_mario_action(gMarioState, ACT_IDLE, 0);
    }
}

This tweak was originally posted by “Revo” in the N64 Decompilation discord.

To skip the mario head sequence when you start the game, change

exit_and_execute /*seg*/ 0x14, /*script*/ _introSegmentRomStart, /*scriptEnd*/ _introSegmentRomEnd, /*entry*/ level_intro_entry_2

to

exit_and_execute /*seg*/ 0x14, /*script*/ _introSegmentRomStart, /*scriptEnd*/ _introSegmentRomEnd, /*entry*/ L1

in levels/intro/script.s at line 20.

This tweak was originally posted by “Revo” in the N64 Decompilation discord.