Tip: If you place a moon, midpoint, or dragon coin through Map16, it will always respawn whenever you reenter that sublevel. It will not respawn if you add it through the Add Objects/Extended Objects window.
Vanilla 1UP point checkpoints:
Super Mario World has four 1UP checkpoints objects. They don't appear if you already collected them all in a level (even if you die), so they can be used for checkpointing. Essentially, you can hide blocks (for instance warp pipes) by putting 1UP checkpoints over those. Then, when time comes you can guarantee getting 1UP checkpoint (by putting multiple 1UP checkpoints in a row).
Display Level Message 2:
Put two Display Level Message 1 sprites in the same place.
Inverse switch palace blocks:
To be written, but I somehow managed to do that in vanilla (hint: bottom tiles of palace switches are solid, the object doesn't appear when switch is already pressed, so you can put something below it).
JSL to subroutine ending with RTS (credit: I don't know, possibly Alcaro):
This code works by pushing five bytes on stack (bank, return address, address of byte 6B (RTL) minus 1 (to handle PC implicitly changing) in a given bank. Then it jumps to RTS function, which uses RTS opcode to move PC to RTL opcode, which returns to a provided location
varies table depending on bank (fast ROM banks are identical, so change 8X into 0X):
00 - $84CE
01 - $80C9
02 - $B888
03 - $827E
04 - $8574
05 - $8124
07 - $FC50
0C - $9398
0D - $A104
7X - write RTL somewhere or modify your function to end with RTL