Fuusui Mysteries
Are you familiar with the ancient art of Fuusui (Feng-Shui)? Who would have guessed that it even applies to Nintendo's Animal Forest +? It's all about how you arrange your furniture in your house.

Orange colored furniture in the North
Yellow furniture in the West
Red furniture in the East
Green furniture in the South
What does this do? For example, the yellow furniture increases the amount of money you receive. The other colors have similar benefits, so be sure to follow this basic rule when constructing your house.

Get Rich Quick with your GBA
If you've got a GBA and the GameCube Link Cable, you can access an island to the south of Animal Forest + that lets you make money much more quickly. Collect as many pieces of fruit as you can carry, hook up your GBA, turn it on, and then go to the pier at the beach (all the way in the south). A Kappa will be waiting here in his boat (he only shows up if your GBA is switched on) and take you to the island. Once you're on the island, drop all the fruit you have collected and go back to the boat. Tell the Kappa that you want to leave and that you want to transfer the island to your GBA. Your GBA will turn on and you will have see your island, complete with all the fruit you have dropped. Move your hand to the door of your island animal's house and knock. Your animal will come out. Now grab a piece of fruit and put it right in front of your animal. It will eat the fruit and turn happy. After a few pieces of fruit your animal will drop a money bag of random size for each fruit you feed to your little virtual pet. Repeat this process until all pieces of fruit are gone (you can also direct your pet to shake trees to get more fruit), then put your GBA to sleep. Do NOT switch it off.

Go back to the GameCube game. Once back in your town, leave the screen with the boat and come back. Talk to the Kappa again and the GCN game will be updated with the new data from the GBA island. Travel back to the island and collect all the money bags. Some bags contain as much as 30,000 coins! You should be able to pay off your house in no time now.

Grow a Money Tree
Did you know that besides growing fruit trees, you can also grow money trees? Yep, no joke. However, you can't just grow them anywhere. Find a sparkly glowing spot on the ground (the type that hints at a hidden treasure) and use the shovel to unearth the money bag. Instead of just pocketing the money and closing the hole, stand in front of it, go into your inventory and select the money bag. Now choose the "plant item in ground" option that you also use to sow seeds or fruit and put the bag back into the sparkly spot in the ground. A seedling will sprout, which will grow into a money-bearing tree. Nice, huh?
Note: that planting a money bag just anywhere will not produce a money tree. It has to be planted in the sparkly special spot.

Reset Follies
Don't ever hit the reset switch when playing this game. If you do, the next time you play you will be chastized by Mr. Reset, an angry mole. He will lecture you on how there is no reset button in real life -- so you shouldn't use it in this game either. Mr. Reset's rant takes quite a while to get through, but that's nothing compared to what happens when you hit reset again... His complaints will get longer and longer and he will even force you to apologize (which requires you to manually enter your apology via the letter entry wheels). Not fun. So stay away from the reset switch unless you understand Japanese and want to read some funny stuff.

Win Famicom Games
Animal Forest + features an even bigger variety of Famicom (NES) games than its N64 counterpart. You have to be lucky to get your hands on them though. There are several ways to get new games:

Be the first. The first player to start a character on the included Animal Forest + Memory Card 59 will receive a letter from NCL boss Hiroshi Yamauchi after all initial chores are done. The letter includes a grab bag with three items, two of which are Famicom games.
Enter the monthly lottery at the Tanuki Store on the last day of the month. You get a lottery ticket for every major purchase you make at the store (stationary and plants don't count). Every five tickets let you play in the lottery once, so be sure to collect a bunch of them. Note that lottery tickets don't carry over to the next month. They're only good for the current month. Prizes in the lottery vary from month to month, so don't give up if you don't find any NES games the first time you enter the lottery.
Check your mailbox on Christmas Eve. Every player gets an NES game as a Christmas present.
Get a game for your birthday. Don't only ask animals if you can run chores for them -- use the second "conversation" option as well. At one point, one of your animals will ask you for your birthday and blood type. This animal will drop by your house on your birthday and personally deliver a new Famicom game.
There are other ways to get your hands on emulated classics, so keep your eyes and ears open.