Lemonade would be easy. Or homemade lemon sorbet?

I was also going to suggest using frozen fruit.

For a picnic, you'll want a finger food. I'd suggest summer rolls. They rely on fresh veggies but with a good sauce (I use a peanut sauce), veggies that are only so-so are delicious. Fresh cilantro will also pep them up.

coleslaw, potato salad, strawberry shortcake (with frozen berries)

do the sniff test! smell what 'summer' produce you want to serve at the store, if it doesn't smell like a melon, berry, whatever, it won't taste like one. you can still find good produce in the winter if you put a little extra work into it. i made chocolate covered strawberries on vday for my boyfriend and they were so red, juicy and sweet that you would have thought it was early summer.

But if you're opposed to buying foreign fruit, citrus is in season, frozen works well too.

What about something with pesto? The green green color will bring a little summer to anything.

Punch up your flavors with lemon or lime juice and fresh herbs. Fruit juice spritzers, ice creams with chunky fruit jam. Creamy salads with fresh herbs. Grilled meats etc. Gazpacho?

I agree - coleslaw and potato salad say summer picnic to me, and potatoes and cabbage are fairly in-season right now. Same with young organic carrots - they are popping up in markets right now. Also - gourmet pickles, cold fried chicken, and a strawberry shortcake made with a sauce from frozen strawberries.

You could probably make a great salsa with frozen corn (which is often superior to fresh anyway), canned fire-roasted tomatoes, and beans.
Citrus is summery, despite being a winter fruit.
If you don't want to go with a basil pesto, why not try a cilantro, parsley, or arugula pesto?

Something will grilled marinated asparagus? Our fave at barbeques, and growing now on this side of the equator.

Grilled chicken with shaved coconut, we saw the recipe on cooking channel, a show called Everyday Exotic. Meringue cookies for dessert.

I would say something light with tomatoes. Tomatoes scream summer! I used Muir Glen Fire Roasted Diced canned tomatoes to make a delicious quick bruscetta topping. Drain off tomatoes, roughly chop, add a few glugs of great quality olive oil, red pepper flakes, kosher salt, and a couple cloves of smashed garlic left whole. Let it sit a few hours to really develop flavors.

I also would add fresh chopped basil if you can find it. I like to add it just before serving to retain that beautiful color, but a little added to the mix will add good flavor.

Most times I'll add a little honey or maple syrup to cut through the acidity that canned tomatoes can have. Taste and see if you need to add it.

Serve with fresh baguette or slice and toast it.

key lime pie perhaps since you can just use bottled key lime juice.

I would suggest making a Key Lime Pie. Even if you can't find fresh Key Limes, you can use a bottle of Nellie and Joe's Key West Lime Juice and just garnish with regular ol' limes. To me, nothing says Summer better than a Key Lime Pie.

I totally agree with the comments saying "something with good canned tomatoes." Salsa and bruschetta are good ideas, also maybe foccaccia or pizza with some fresh basil and mozzarella.

Depending on what kind of storage and transport you have, popsicles scream summer for me and are fun, easy and different!

But if you're thinking something less desert-y, why not do a grapefruit salad of some sort?

Banana pops, canned tomato salsa with quesadillas or tacos, ice cream sundae buffet...

I always think citrus feels like summer, even though their season is winter. Lemon or lime squares, lemon poundcake, lemon or lime cookies, or for savory a lemon-marinated chicken would do the trick. Citrus always feels light and fresh.