There are some good mixed salad leaves seeds around which do really well in pots and you get lots of different things - plus a bit of shade is an advantage because it stops things bolting - and you can keep cutting them all summer and into the autumn and more grow. Google Park Seeds - i got a collection of about 5 different salad mix seeds from them a few weeks ago and they were a bargain (although haven't tried them out yet but my mum has tried them previous years and said they were good quality)