My autistic sons birthday is coming up...?

My son is fixing to turn to 4yrs old... hes mildly autistic... he loves the outdoors... he already has a 4wheeler that he doesnt seem to have too much interest in... he loves bubbles.... loves playing in the water and loves playing with trucks/cars... i thought bout getting him one of those motorized trucks he can drive and ride in... but im thinkin its too advanced for him... need some ideas... thanks