Boosting stats/crit/mov for 1-3 turns from an item that's hidden in specific tiles is super easy, it's just an item that applies a status to the user. Status also, of course, causes an effect to play on the map sprite, and tints the battle sprite. To make it rainbow, change music, and add another action, you'll probably need to code that in. Rainbow and music aren't gonna be too hard to code if you're crafty at re-using systems already in place, but adding another action is gonna take a little more know-how to make reliable. Far from impossible, of course, but there are various other considerations you'd have to make in doing that, too, like how the extra action is conveyed to the player blah blah blah