branpatrobinson:
HazardsMind, thanks.
Like this?
laser
Yes, though you can get those lasers quite a lot cheaper on eBay (or perhaps not, depending on the power).
Now, some practicalities. These are red lasers, and like the one shining on the floor next to me, project a very visible spot. In fact, those I have use a 91 ohm resistor and can be driven directly by an Arduino pin whereas the ones you cite - if the picture be correct - have a lower value resistor and thus are 5 mW rather than 1 mW, so will be even brighter. This means that not only will you get the stairs lighting up, but brilliant red spots flashing about on the legs or feet of those climbing.
Though you were concerned about attaching things to the steps, you now have the problem of mounting things above the ends of the treads which will be equally challenging.
Were you to do so, I will make one suggestion. These lasers are current-controlled by a single resistor only - which works fine for a 5V supply and also means that you can modulate them at almost any frequency, so you can for example, pulse them for a tenth of a millisecond, ten times a second which would be adequate for detecting feet stepping, but reduce the visible power substantially. I suspect you would then need to use photodiodes rather than LDRs (which are really slow) to get the necessary response time, but I may actually do some experiments along those lines.