You can probably grow different salads all year round. I had an article on this somewhere, I remember trying it for a year and it worked (ie no need to buy salads at all for a year). The drawback was having to have a bunch of different seeds, and the right ones at the right time. Roughly, you can grow different types of lettuce in spring, summer, autumn, then endives (or chicory, I can never get these names right between British, French, and American usage), including the red Italian kinds that heart in January.

A nice catch crop is corn salad, there are autumn varieties and frost-resisting varieties for later in the year. Rocket should be easy to grow in your climate, spring and autumn. Sorrel is a bit acid, but can go into mixed leaf salads (and also be cooked like spinach, accompanies fish well). Red cabbage is good raw, sliced thin, and easy to grow.

Spinach would be good, too, and chards which you've already mentioned. Beetroot you could have over a fairly long season. Spring carrots and onions would be nice, even if you chose to buy in for the rest of the year rather than take up too much space.

It might not be done where you are, but I'd want to sow peas and broad beans late in the year to get them out of the way a bit earlier the following summer. Cabbages, Brussels (Euro)sprouts -- I don't know, they take up space and the growing season is long.

Tomatoes -- we grow "heirloom" varieties and have done some testing of these with our nurserypeople neighbours. Here we get a long season, so we've tended to select and keep (seed from year to year) of long-season varieties. But there are Siberian (!) short-season varieties we've tried, and they're good. I'll have a word with the neighbours and see what they suggest.

One variety I'd definitely recommend is Bloody Butcher -- the fruit is small to medium sized and bright red, very tasty, and it starts early and will go on till the season is over.