Please visit my Discovery Toys website! The toys are very high quality and they all come with a lifetime warranty, so if anything breaks through normal play, you get a replacement at no cost to you. Plus, the toys are educational, so the kids will learn while playing, but they are so fun that they won't realize they're learning!
Now, for specific recommendations:
Motor Works - this is our #1 seller for boys this age and I LOVE IT! My son is almost 5.5 and has been playing with it for three years. He still loves it. You get a toolbox with a car, a motorcycle, an airplane, and two screwdrivers - one battery powered and one manual. You can build, take apart, and rebuild the vehicles over and over again. The kids build fine motor skills, thinking skills, and problem solving skills. They also enhance their imagination and creative play.
Marble Works - an awesome marble run toy with endless possibilities for creation. The boys can race their marbles down the tracks, predict the outcomes, build their hand-eye coordination, and so much more. Available in starter and deluxe sets. Note: this does use regular marbles, so it isn't recommended if your three year old still puts things in his mouth.
Games on the Go - set of 50 cards, each with a unique game for playing in the car, at restaurants, or anytime you're on the go. Games may be trivia, guessing, memory, thinking, searching, and more!
Race to the Treasure - a fun board game where you lay down track pieces to collect keys and beat the ogre to the buried treasure
Playful Patterns - 132 foam ships and 34 increasingly difficult design cards to create pictures and mosaics. Build math and reading skills, as well as enhancing fine motor and thinking skills. We dub this one "preschool in a box" because it prepares your children so well for the skills they need to learn reading, math and more.
Your six year old will also like Wiz Kidz and Number Rings. You can view these and all of our other toys on my website at http://www.discoverytoyslink.com/karenchao
Please let me know if you'd like info on getting your toys for free!