My nephew at that age was delighted with pretty much anything with wheels on - train, car, aeroplane. The ELC do have some nice chunky ones that come in sets (race cars, construction vehicles, emergency vehicles, etc), and they have smiley faces, and join together with magnets like a train. At 2, Oli had worked out about the polarity of the magnet needing to be right for them to stick together. Might not be a small enough package though. I don't think they sell them singly.
Or if they already have the generic wooden train stuff (Brio, or own brand, they are usually interchangeable), a new loco always goes down well. Bigjigs is the main generic brand, and much cheaper than Brio - but it does depend on them having a set already, unless you are prepared to buy a starter set. I think they can be found in many toy shops, as well as online.
http://www.bigjigstoys.co.uk/bigjigs-rail-wooden-railway.html
I think even Tesco have own brand sets, pretty cheap.
Basically, it needs to be playable with instantly, as summerdays says.
Depending on the child, story books are a good back up. Oli often takes to a new book, but later, once the exciting present has been played with, or at bedtime. So something for him to play with, and a nice storybook to give to Mum or Dad to produce later.
If you can get any clue about what his current craze is, that might help. A cheap plastic dinosaur might turn out to be worth a thousand expensive trains.
And I don't know how much time you have to look, or how you feel about the idea, but charity shops can be a good source of cheap pressies. At that age, quantity matters far more than price!
