what do you want the microswitch to do?
if you know to program for AVRs, you can even switch the bot from photovore to photophobe mode, or to other custom function.
otherwise, you can do little with switches. one idea is bumper switches, as ed1380 said. bot bounces around, when it hits something with the switch, it turns (or hits it again, s/m mode
you can put another switch on the battery pack, for example to have an on/off switch