I will give the Roomba Create the ability to perform SLAM (simultaneous localization and mapping) within a typical house setting.
There are three parts to this project:
1) By using several IR rangefinders mounted on servos, create a 2D map real-time of the surroundings. This will be done with zero modification to the Create chassis. Very easy.
2) Upload the map by wireless link to a PC for analysis. Intermediate.
3) And the hardest part, continuously update the map accounting for drift over time (SLAM). Difficult but very useful!
And I will give myself one additional challenge - I cannot use more than $100 in parts! The average hobbyist cannot afford much more than this, so to keep my tutorial useful I will stay within this limit.