I've concluded from many years of building with kids that when it comes to construction kits (Erector sets, K'nex, Tinkertoys, Duplex, Legos, and so on) long-lasting enjoyment is proportional to the simplicity of the pieces.

Almost nothing is simpler than Kapla blocks. Simpler even than Legos. There is only one size, one shape, no holes: a sturdy wooden plank precision milled from hardwood. The key difference between these and ordinary wooden blocks is something they share with Legos: amazingly precise dimensions. Because of this machine-like uniformity you can build very large, high and stable structures. Even little kids can build very rapidly (without the planning Legos seem to require) and very large. And because of their simplicity -- one simple plank -- you can build with endless variety. Finally, because they lack the tricky locking device of Legos, and are not tiny, kids of the youngest age can build with them. And you can't break them if you tried.

Kapla blocks come in wooden buckets of various multitudes. The 1,000 piece set is sort of over-the-top, but endless fun (suitable for a dayschool or the like). Like the best toys, these are not just for kids.

I would include Kapla blocks, along with Legos, as a toy that you can pass on to the next generation.

-- KK

Kapla Blocks
$53 for 200 piece set

Also from Amazon

UPDATE: It appears Kapla blocks are no longer available in the US, but there is a very good equivalent in Keva Planks. These are manufactured to the same size, out of maple, and are about the same price as the old Kapla blocks.

Keva Planks
$68 for 200 piece set
Available from Amazon