Simple Toys...

... that I would add to this list (via Instapundit) based on personal experience:

• The Original Rollercoaster by Anatex
• Fisher-Price: Doodle Pro
• Djeco Puzzles - We have the wooden panda and the wooden cat. Both have been big hits.
• Schleich Animals - No other animal figures that I know of are as detailed and durable.
• Haba Blocks - These wooden blocks cost more than others, but if your kid likes to build tall, these are ideal because the surfaces are perfectly flat and the angles are dead on. Some other blocks will give you leaning towers after a certain height.
• Colorforms

ADDED: I almost forgot about this:

• Little Tikes Push & Ride Racer - One of the most beloved of toys. This one is great because our son prefers it to all the other, more expensive, toys of this type.

And here's what I mean by building tall:

You can't do that with blocks that aren't cut just right because they'll fall over before you ever get that high.