I've just been messing with ZSNES' cheat search function, and came up with these PAR codes:
7E0D70XX - Force an item to be used instead of what you collect. Replace XX with:

• 00 - Mushroom
• 01 - Feather
• 02 - Star
• 03 - Banana
• 04 - Green Shell
• 05 - Red Shell
• 06 - Ghost
• 07 - Coin
• 08 - Lightning

Values above 08 cause odd things to happen by using multiple powerups at once. In some cases it gets a bit confused.

1B - Banana, Feather, Ghost
Feather causes a big hop instead of a jump.

47 - Banana, Star
You get all the effects of the Star except for the invulnerability, and it lasts infinitely. This is the simplest of the many values that cause this to happen. Use 0C to add a mushroom as well.

62 - Red shell, Lightning, Ghost, Mushroom, Star
The most insane combination I could find.

77 - Star, Mushroom
Nothing special, just a cool combo.

7E0D71C0 - Use the above item infinitely. Time for revenge, Mario! Mwaaaahahahahahahaaaaaaa... ha... ha... err... hello?