Oil: Combine oil, garlic, salt, pepper, and red pepper flakes. Lightly brush corn and cut zucchini with ¼ cup of the mixture (keep the remainder).
Grill: Grill veggies over medium/high heat, flipping occasionally to evenly cook, and charring some (but not all) for flavor. The zucchinis are done when they're a touch soft, about 10 minutes total. Corn may take a little longer, about 15 minutes, and is done when bright yellow with charred spots.
Cut: Cut corn from stalk (it's hot, so you may need to grasp it with a paper towel). Cut zucchini into ½ inch wide pieces.
Assemble: Whisk lemon juice into remaining oil mixture. Stir in grilled veggies and feta cheese (amount to your liking). Serve warm or cold.
Store leftovers in an airtight container in the fridge for 4 to 5 days. Great hot or cold!