Nitro Planes has a well priced, Arduino-compatible robot kit available. Once assembled, the four-wheeled Remote Control Blutetooth Ultrasonic Ranging Smart Car Kit can roll over grass, gravel and sand. It can even climb a sloped surface.
The kit includes an ultrasonic module, one infrared receiver, an MCU remote control, a chassis and rubber wheels. You’ll need to add your own Arduino, of course. But plenty of sample code is available online and could be easily adapted to work with this bot.
Nitro Planes is currently running a sale, so the kit costs just $99.95.