For the important items, if you are going to make codes for it use the address ranges 02B07C80 - 02B07CD6 (16-bits/item). The full range is 02B07BFE-02B07CF0, but if you accidentally overwrite a needed item, bad things can happen. ;) Not all important items will show up in your pause menu (like the earlier hammers), but they are still in the important items range. That is why I suggest using the above range.
For the Item, Stored Items, and Badges codes below, I suggest loading the game with the code, saving the game, then restarting the game with the code off. Alternatively, you could make the codes button-activated.
WARNING: Do not put important-type items (Mail Box SP, Diamond Star, etc.) in the regular items slots. Otherwise, you won't be able to remove them. If you do anyways (or just want to try it out), use the "Empty Items" code below.