Books have been our greatest tools so far. We recently got Explorer: A Daring Guide for Young Adventurers (http://www.amazon.com/gp/product/0763636487/ref=oh_o02_s01_i00_details). It has been an awesome source of ideas for her. DD (7 next month) uses it as a jumping off point for finding things she wants to know more about, and the tips for being a young adventurer have really given her imagination a boost. She's now requesting help with researching Ancient Egypt, and wants to know everything about King Tut.
She loves the Ology series as well. She just got Bringing Up Baby Dragons (http://www.amazon.com/Dragonology-Bringing-Baby-Dragons-Ologies/dp/0763636525/ref=wl_it_dp_o?ie=UTF8&coliid=I2HK9EZ0M2BYHD&colid=3PXHNXME3YZVQ) and has had a lot of fun with it. I plan on getting her the rest of the Dragonology books soon. Right now she's working on her drawing and creative writing skills to document the imaginary baby dragons she's rearing.
Faeries: Deluxe Collector's Edition (http://www.amazon.com/gp/product/0810995867/ref=oh_o02_s01_i01_details) resulted in the idea to start a fantasy novel together. We've been working on it little by little for the last couple of months and she's very proud of it.
Children's encyclopedias have also been really great for helping her to find interesting subjects to look into.
Books aside, gardening is a pretty strong interest right now. She's been tending a couple tomato plants on her own, and they look great. She has a journal she uses to document what she's doing to care for them and how they're changing. She's also really into insects, and likes to observe them. Sending her outside with a magnifying glass, a mesh "observation house," and a notebook keeps her happy for a good while. I'm planning on setting her up with a worm farm, an ant farm, and a praying mantis soon.
Tools for exploring the world around her are invaluable for us, though we don't have much yet. I've done a lot of scrounging around, and made what I could to come up with an exploration kit for her. For awhile she's been asking for a microscope, a telescope, and a chemistry set. I know she'd get a lot of use out of those, so I'll try to come up with those soon.
I definitely agree that having a special work space set aside can encourage a child to pursue their current interests. Getting her own desk isn't practical right now, so I cleared half of mine and declared it her personal work space, and boy does it see a lot of use. Also, she has a case with a handle to carry her notebooks, pencils, and various exploration supplies to serve as her on-the-go work space.