German Riesling: Classic Midwestern picnic wine, these run the gamut from sweet and fruity to dry and minerally. They're also incredibly food-friendly (the sweeter varieties work well with spicy foods like Thai and Indian). Guntrum Riesling Royal Blue is a great dry variety; Schloss Vollrads Reisling QBA, Rheingau is sweeter without being cloying.
Vinho Verde: This light, airy, and half-sparkling Portuguese wine is one of my favorites as an everyday substitute for champagne (and no headaches later). Terrific with salads and seafood, the crisp, green-apple flavor is not too sweet, not too dry. Try Sogrape Vinhos Gazela, which has a tinge of lime in it, or Broadbent Vinho Verde with its subtle apricot hues.
Gamay: Thought I was only going to mention whites for summer? Burgundy's other red, Gamay, is a lighter cousin to those thick, velvety reds that work so well around Christmas time. Lightly acidic with berry notes, serve this slightly chilled at picnics with everything from salami and hotdogs to salads and grilled chicken. Try Georges Deboeuf Morgon Jean Descombes, Cru Beaujolais, France, which is the definitive Gamay available in the states. Maison Louis Jadot Beaujolias Villages is another option available locally; I like it for its bright currant flavor and peppery finish.
Syrah: Long equated with budget Australian labels (which is a nice quality, I think!), Syrahs can work with heavy, wintery foods and light, grilled foods equally well. This is the red to serve with your grilled pork, lamb, or beef. For the lighter side of Syrahs, try Bonny Doon Le Pousseur orArnot-Roberts Clary Ranch, both from California. They have a gentler finish than the Australian varieties often do.