A delivery robot is easy to prototype and hard to make street-legal. The realistic DIY version operates on private property: campus paths, warehouses, farms or a driveway. Keep speed low, use teleop fallback, and do not assume public sidewalk operation is allowed.
Mechanically it is a rugged rover with a lockable box. Two wheelchair-style geared motors give enough torque, pneumatic tires handle rough paths, and the cargo box sits low between the wheels. A front bumper is mandatory because GPS and ultrasonic sensors miss real obstacles.
Navigation can start with GPS waypoints plus manual override. For more autonomy, add a depth camera or 2D lidar, but the first milestone is reliable remote driving and return-to-base. The robot should stop on comms loss, not continue toward the last waypoint.