Whole rows of plants can be covered using long, low tunnel cloches, which are left open at the ends for thorough ventilation or closed off when greater protection is required. No rain will reach cloched plants, so remember to water them as necessary.
If I could only pick one season extending structure, it would be a cold frame. Incredibly versatile, a simple cold frame is the key to taking your garden from a two or three-season plot and turning it into a year round vegetable factory.
A "How-To" on using pvc supports over your raised beds, cover them with plastic, cheesecloth, or wire to make a green house or keep out critters. Idea: Drill holes in them and have it hook to your hose?