My 4 1/2 year old has always been interested in trains and still is, so we have a lot of tracks around our house (different kinds, too, like wooden, flexi, tomica, etc). I like how they not only fill his need for train play but fuel his imagination and math skills as he designs track layouts and builds things for them to go past.
He is also obsessed with Legos now! A lot of the "city" series is for kids 5 and up and he is able to follow the directions and create a ton of things on his own, too. He also likes the "creator" series for 7+ if we help the first time, then he is good mostly on his own. He also still plays with the Duplo/MegaBlocks bricks if you don't want the little legos yet.
Staying on the building theme, he has a renewed interest in his basic wooden block set (the 100 piece one from Melissa and Doug). Today alone he made his uncle's house, a grocery store, the empire state building, a water tower, playground, library and beach house for a city in the living room.
Airplanes and rockets are a hot topic right now and we dug out my brothers' and husband's old ones and it's like our house is the world's busiest airport.
He spends a good deal of time drawing with all sorts of things like crayons, paint, chalk, glitter glue, markers. We have a little table set up for him that has all of his art supplies (except the paint) in a bucket underneath and plenty of paper at hand.
Games are wonderful for us to play as a family and he is also old enough to play with friends and no adults now. Your boys might enjoy Hisss, Go Fish, AnimaLogic, Bingo, Crazy Eights, Twister, Operation, I Can Do That, Dominoes, Busytown, Hullaballoo, Lite Brite, Bean Bag Toss and Pop-Up Pirate. Even an age-appropriate chemistry set fills our time.
And books. Books. Books. :)
There are so many things out there it is easy to get overwhelmed/get too much. See where your boys interests are and go from there!