There's a game available now from Gamewright called Frog Juice. I picked it up and discovered that the cards are beautiful, but the actual game is, well, rather dull. I therefore designed my own game that can be played with the Frog Juice cards but is otherwise completely different. I call my game Monkey Powder.
To win, you need to cast as many spells as possible. Each time you cast a spell, you will set the components aside to be used later in determining your score.
Setup: Place the 5 spell cards in the center of the table so that all players can see them. Shuffle the rest of the cards and deal 4 cards to each player.
Draw cards: On your turn, draw cards as needed to bring the number of cards in your hand up to 5. You may draw cards from either the top of the deck or the top of the discard pile.
Play cards: If you have the right cards to cast one of the spells, announce that you are doing so and place the cards on the table in front of you. These cards go into your "Points Pile", which will be used at the end of the game to determine your score. If you have the cards to need to cast more than one spell, go ahead. You may also use the Black Cat or a Princess card during your turn (see below).
Discard: When you have finished casting spells and playing cards, place one card on the discard pile (unless you have no cards left in your hand).
If you look at the recipe for the Prince to Frog spell, you will see that it instructs you to combine some Frog Juice with Shrinking Brew and then add one Prince. You can therefore cast this spell in 2 ways: either just mix up the potion and add it to your Points Pile, or go the extra step and turn the Prince into a frog, putting the Prince into your Points Pile as well. But watch out! If another player has a Princess, she might suddenly appear and whisk the Prince off to her Points Pile!
Most of the cards are used as components for casting spells, but the following have special uses:
Witches: You may play a witch card at any time another player is casting a spell. The witch flies in and steals the components of the spell just as it is being cast. You put them into your own Points Pile as if you were the one who had cast the spell.(The witch card also goes in your Points Pile, although she isn't worth any points.)
All Purpose Witch Wash: This can be used to stop a witch from stealing your spell. When you use it, place both the witch and the witch wash card in your Points Pile, even though they aren't worth points.
Princess: The Princess can be used in either of 2 ways: You can imprison her, or you can use her to rescue a Prince. To imprison her, simply declare that you are locking her up in the tower and set her in your Points Pile. (Watch out, though, if a player has a Prince, he might rescue your Princess.) You can also use a Princess card on another player's turn, to keep a Prince from being turned into a frog. As soon as someone else casts a Prince Into Frog spell (with a Prince), show the Princess card. Your opponent still gets to put the Frog Juice and the Shrinking Brew into his or her Points Pile, but you take the Princess and the Prince and place them in your own Points Pile. Note however that the Princess must be played right when the spell is cast... after the spell is done, the Frog Prince hops off to the forest and is gone.
The Prince: As explained above, the Prince may be used as part of a spell. However, if another player attempts to imprison a Princess, and you have a Prince in your hand, you may rescue that Princess and play both the Prince and the Princess in your Points Pile. You have to do it right when the player is imprisoning the Princess though... if you wait until after the Princess has been whisked off to the tower, you'll never be able to find her.
The Black Cat: The cat is a sneaky embezzler. On your turn, you can use the cat to look at another player's hand and take any one card that you like. Add the card you steal to your hand. Then place the cat on the BOTTOM of the discard pile.
When the draw pile is exhausted, reshuffle it together with the discard pile and keep going. As you proceed, there will be fewer and fewer cards left to do this with, but keep going anyway. You'll reach a point at which there aren't any cards left, and each player can draw only the one card discarded by the previous player. At this point, the game is almost over, but keep taking turns until no more spells can be cast. You should be able to keep going until the only cards left are the Black Cat and possibly one or two lonely Princes, although occassionally it'll end in a stalemate.
When the game is over, each player should add up the numbers on the cards in their point piles. The player with the most points wins.