All kid love building things. Although it seems in my experience that the real fun bit is knocking them apart afterwards! I've had Mega Bloks playsets for my son since he was 18 months old. Play.com have this Thomas the Tank Engine set, Thomas at the Sodor Fair, on half price offer at £14.99 including delivery.
Now it gets a bit complicated if you're buying Mega Bloks toys. In the UK it seems like all their products come under the Mega Bloks brand. However, there are two different types of brick involved, and they aren't compatible. One is the traditional Mega Bloks building bricks, aimed at toddlers, with large robust looking connectors. The other type, which this set belongs to, looks pretty much like Lego. Just less fiddly and with sturdier and chunkier accessories.
In my experience, these have been better suited to younger children who are too old for the building blocks, but too young for the fiddlier and much smaller Lego figures and accessories. For example, the little Lego figures can all be pulled apart and often have tiny removable hats, and handheld items. Which frankly, a four year old will soon lose. We've had a number of these play sets, including ones from Diego and Dora the Explorer, and they've survived quite a hammering. Unfortunately you can't use Mega Bloks with Lego, so if you've a child with both it is well worth your sanity to keep them in seperate tubs!
This set comes with a decorated Thomas, a working ferris wheel, blocks, fair tent and ramps. The recommended age is 3 plus.