Cheap and cheerful....put a series of "typcial" scripts (select + updates, etc) together that can be randomly called by some utility....possibly either off the schedule or via multiple client sessions.
For randomness you could put them all into one procedure and use some element of the system-clock to branch and execute different items.
There are also some load/stress testing utilities out there .... haven't dug too deep myself into this arena (out of lack of necessity) .... but a google search may give you some leads.