I also know exactly what you mean. I have a tendency to also get migraines easily from certain scents. I also have really bad allergies, and find more potent scents to be aggravating; especially in the morning before taking my allergy meds. Sometimes even when my allergies are at bay I just like to wear something light.
Clean, Philosophy, and Escada are all fabulous long lasting crisp scents! Clean's Outdoor Shower Fresh is what I'll typically wear if I'm running errands or hanging out casually with friends. It has a very beachy scent to it which makes it an great summer scent. My favorite Philosophy scent is Amazing Grace, which is the perfect light blend of florals and musk.
I find that Juicy Couture is another scent that is on the lighter side (which I'm wearing today). The gentle notes of vanilla, princess lilly, and creme brulee give this perfume a muted warm scent. It's soft yet very charming and romantic.