Plant cool season seedlings or seed in early- to mid-April. Cover if hard frost is predicted, or use row covers (recommendation below).
Best cool-season veggies:
Arugula - does well all throughout the growing season and is a wonderful addition to salads, on top of pizza, and as a savory edible garnish. Has pretty cut flowers if allowed to bolt in summer heat (or keep cutting it back if you don't want it to bolt and it will continue to produce wonderful tasting leaves). Turns out arugula is a magnet for tiny flea beetles, which keeps that pest from your other crops (silver lining!). A quick rinse before eating is all that's needed to clean off the little bugs.
Sugar Snap Peas - give them a climbing structure and these will give you lots of pea pods for stir fry recipes as well as peas (if left to grow the pods to fullness). Snap peas just keep on blooming and giving in the garden, and while mine slowed down in heat, they did continue to bloom here and there and form pea pods for snacking while gardening.
Lettuce - plant three or four different varieties including mesclun, butter lettuce, and red leaf lettuce. Put row covers over the lettuce for protection from frost and those unusually hot spring days.
Kale - Kale is a cold-hardy crop that can be planted in early spring or late summer. Cool weather brings out the sweet, nutty flavor of this highly nutritious leafy green. Our only problem with kale was cabbage aphids, which was a pest of epic proportions. Click here for a full article on cabbage aphids.
Spinach - like the superfood it is - easy to grow and gives a prolific bounty of dark green delicious nutritious leaves for months and months. Plant something near it that will provide shade in the heat of summer if you can (i.e. tomato plants or arugula which gets 16 - 20 inches tall), and you'll likely have produce from your spinach plants all the way through summer. Succession plant in early fall.
Row covers: I use and recommend these row covers for cool season crops. If cleaned and stored they'll last for years. Because of high winds, a brick/heavy rock or two on the top of each keeps them from blowing away.