My kids received the Tulip Tie Dye kit a couple of years ago and it worked pretty well. They had a lot of fun with all the different colors and patterns they could create. This is a good reminder that I should pull it out and let them use it again.
Rit has some great dye too, but if you're just playing around go with a tulip kit. If you want to do thicker fabrics like hoodies or stuff like that make sure you really soak it in dye. I have a hoodie that got beautiful color all over the front, arms, and hood but the back looks like someone spilled juice on the front and it soaked through in spots on the back :/