The idea I'm sharing here is how I made a compact simple to control robotic platform. I feel that showing you what to do with it exactly would not have much of a point, you can do whatever you like with it.
That said, I'll make some suggestions of how you could expand this robot without reinventing the wheel. Other Instructables are suggested to get into the low level details of these features, and links are provided to buy them where available. I didn't make these Instructables, but they will integrate nicely, that's the great thing about an online community after all:
The easiest way to add a gripper is with another micro servo, one that isn't modified for continuous rotation. You would add its control to code simply by attaching another servo, and giving it a position command, as seen in step 6.
Here is an example which would fit right on:
The same method could be used to add a scoop or other manipulator as well.
If you want to just buy one, something like this would work nicely:
Jameco 1.3 inch gripper
A fun thing to add to any robot, there are piles of tutorials on how to do this with an Arduino.
You can do it with blue-tooth (easy but expensive)
Or you could use an XBee (easy, and less expensive)
Since the XBee is wider than the Arduino nano, and can't plug into it, I would actually suggest setting it over the nano and wiring it in around it.
Examples of the code needed are widely availible, heres an XBee library for arduino
Of course, you could get an XBee/Arduino nano board, like this one
from robotshop, and your robot would have a swanky tail.
You can do it with a bare transmitter receiver (cheapest, but requires knowing what you are doing):
Detect movement (PIR)
This is actually remarkably easy, a PIR sensor compares the infrared map of its environment with one it has built over time. So it detects changes. They can be found for $10 at several stores including sparkfun
These have everything integrated, and the output pin goes high for a few seconds when motion is detected. All you do is give it power from the battery and connect the output to an input pin on the Arduino. Then set an interrupt or check the state of the pin. As seen on the customization of the platform above. This lets your robot react when someone approaches.
Ultrasonic Range finding
Ultrasonic range finders are an inexpensive way detect the distance in one direction with decent reliability in the range of about 0.1 to 10 meters. They tick out ultrasonic pulses at about 10Hz and detect how long they take to return. Most pick up obstacles in a conical span, so they can be fooled by things that aren't really in the way. It's no LADAR or computer vision, but it costs 1/1000 as much.
This is also on the robot customization shown here. I used an HC-SR04 range finder and this library
. I just got that from ebay for $6.
There is also support for the PING range finder: http://arduino.cc/en/Tutorial/Ping
And even if you use the cheaper one I used, this may help you understand how it works
If you want to give your robot a sense of its/his/her place in the world, there is example code to use a parallax GPS module on Arduino Playground here
I'm planning a few fun projects with these. One will be to add wireless cameras, remote control, and little grippers, and then make tiny obstacle courses for them to explore in teams.
Another fun project would be to mount a Kinect on the roof and make a bunch of these with wireless act as a swarm in formation.
In general I'm going to use these as a basis for lots of projects, and I will be going into more detail about possible add-ons in that way. I would be very pleased to see this used in other Instructables and expanded. After all, sometimes you want to use a robot, but don't want to write about developing the basic stuff.