I agree that the gantry crane is one of the best accessories, but I would consider something simpler for a 2 to 3 year old, especially if he will want to run the accessories under your supervision.
Consider the operating barrel loader and operating barrel car, and/or the operating log loader and operating log dump car. I had them both from the time I was a young child, and loved playing with them supervised by my dad until I was 8 or 9 years old, and then on my own thereafter. Arnold