Science for the very young?
zoba: I think anything that "looks cool" will be good for getting a kid interested in science.  Once you've got him/her hooked, then you can start on the actual scientific method.  To that end, science things that look cool:Cymatics:
http://www.youtube.com/watch?v=1iXY2BE1S8QFerrofluid:
http://www.youtube.com/watch?v=zpBxCnHU8Ao
http://www.gaussboys.com/ndfeb-magnets/FerroFluid25Non Newtonian Liquids:
http://www.youtube.com/watch?v=S5SGiwS5L6I
http://www.youtube.com/watch?v=f2XQ97XHjVwMicrocontrollers:
http://www.radioshack.com/product/index.jsp?productId=211799...  (maybe not the best for a 5 year old, but in a couple years)Make a Speaker for cheap (haven't done this one myself):
http://www.youtube.com/watch?v=8m8fbnShPcwElectromagnets:
http://education.jlab.org/qa/electromagnet.htmlFinally, one project I did with my little brother that I thought was cool.  I got a frequency analyzer for my computer ( http://www.relisoft.com/Freeware/freq.html) and then filled  glass cups with varying amounts of liquid.  Then we ran our fingers around the lip of the glass to get it to "sing" and measured the frequency.  We were able to come up with a function for X amount of liquid gives you X frequency.  I thought this was great because: it was really appealing to my brother (he was 10 or so at the time) because all kids like making cups make noise, we got to do scientific method (hypothesis being more water in the glass) will make a lower frequency, I got to teach him about graphing, how to get a forumla for a line on a graph, and finally we could use that line to predict things to see if we were right.