Heat a broiler on high.
Place the shrimp and lemon wedges on 8 skewers, alternating them. Halve the squash or zucchini lengthwise, then cut them into 1-inch-thick slices. Divide the squash among the remaining 8 skewers. (If you don't have skewers, place the individual ingredients on a baking sheet.) Place the skewers on a foil-lined baking sheet. Brush the kebabs with the oil and season with the salt. Broil, turning once, until the shrimp are cooked through, 3 to 4 minutes total. Transfer the shrimp-and-lemon skewers to a platter. Continue to broil the squash until tender, 3 to 4 minutes more. Serve the shrimp and squash with the basil, torn into pieces if desired.
Tip: For a little more money, you can buy shrimp that are already peeled and deveined, though cooking them with the shells on retains moisture and flavor. To keep the shell intact while you devein the shrimp, use scissors or a knife to cut the shell along the vein up to the tail. Then use the tip of a knife to remove the vein.