When our daughter was three, she's now five, she asked for a plant with flowers for Christmas. She has been very responsible with watering it (with reminders), and we looked up how to take care of it. We gave her money, and she the fertilizer herself. It's been great.
Last year was the most consumerist Christmas when she asked for an Elsa dress. This year she asked for ocean books and little ocean figures from us, and wants a mermaid doll from Santa. I asked her if she wanted anything else, and she said that when we back to NJ (we're currently living in Germany until February) she wants to go see the new microbe exhibit at the New York Natural History Museum.