If you need a steady supply of various kinds of lettuce and leaves for your salads – and let’s face it: who doesn’t? – then why not build your own lettuce machine* ? Ok it may be a bit bulky for use on the balcony, but it’s a clean and manageable way to grow stuff. It’s portable so you can put it indoors at nights, and it’s elevated too in order to keep most icky bugs away, without your having to smother the lot in pesticides. Besides, it’s better for your back as well.
Needs some handiwork though. A challenge 
* lettuce machine website brought to our attention by kichus