There are some items around the castle and paintings, which are Blocks, Switches, Star Coins, Spinning Hearts, Koopa Shells, Cannons, Power Flowers, Rabbits, Green and Red Mushrooms, Keys, and Power Stars.
Keys: There are 3 keys hidden in the painting worlds you must get to unlock Mario, Luigi, and Wario. The paintings will show a picture of these characters. However, to obtain these keys, you must defeat a certain boss. At the beginning of the game, you have to catch a Yellow Rabbit with a key in order to unlock the castle door.
Yellow Power Stars: There are a total of 150 Power Stars. 120 Power Stars are found inside the painting worlds. 30 Power Stars are hidden inside the castle. When you are in the final battle with Bowser as Mario, and he is defeated, he will drop a big Power Star, larger than the normal ones, and even shines. When you jump to get it, you have finished the game. NOTE: This page will be edited soon and made into separate pages.