You can get items such as stationery, non-native fruit, and tools such as a bug net, fishing rod, or a watering can, from Isabelle! Just ask her for advice.
The first time you ask Isabelle for advice, she will give you some stationery and recommend that you send a letter. The next time you ask Isabelle for advice, she will say that she can explain more about letters, or give you some new advice.
If you ask for new advice, she will talk about wanting to go to the beach and look for seashells with you. After this, you should go get a seashell from the beach and bring it to Isabelle. She'll give you a basket of non-native fruits in return.
Her next advice depends on whether you're carrying a shovel or not. Isabelle will explain how to plant trees.
If you ask her for more advice, you can tell her whether you are interested in fishing or bug catching. If you say fishing, she will ask you to catch three different types of fish and then show her your encyclopedia. If you say bug catching, she will ask you to catch three different types of bugs and then show her your encyclopedia.
Talk to her again if you don't have fishing rod or bug net and she will sell you one at the standard price of 500 bells.
Once you have caught 3 different types of bugs or 3 different types of fish, go back and show Isabelle. She won't have any more advice for you after that, so she will give you a watering can as a welcoming gift. After that, she'll tell you to take it easy for the rest of the day.