Hopefully this is the correct place for Secret Santas.
I may be biased but I think everyone should have their own small mobile robot - just for fun. Farnell provide a decent kit chassis suitable for this.
This one has 4WD, several platform areas and holders for sensors - all good things for a mobile robot
But! a mobile robot without sensors isn't much use, so maybe I should suggest a sensor as a Secret Santa gift. Such as this one:
An optical 3D distance sensor operating pretty much in real time. Is it a tool? Is it a gift? I think it is both.
And the price? £34.69 for the chassis and £16.14 for the sensor (a bargain), making £50.83 which when you convert to dollars must be well less than $50. And if it isn't then I suggest the optical 3D sensor as it looks kind of fun, interesting and challenging.