Item 1 – I/O

Sufficient output to be “interesting”

Motors – move, dance, wiggle
Audio – At least beeps and boops, possibly recorded audio, but no extra audio hardware (like mp3 decoders)
RGB LED Eyes – Maybe even moveable (if I have extra time on my hands)

Sufficient input to be interactive

Location sensing – using dual color sensors and a colored mat
Light sensors – need to know when it’s dark out, follow a light or find a dark corner
IR obstacle detection – don’t want to bump into stuff
IR homing sensor – to find the recharge station
Hall Effect Sensors – locate and play with toys
Capacitive touch sensing – how else is it going to know when it’s being petted?