Just a heads up that elderflowers will be blooming in much of the U.S. soon (or are now). We're in MN and it will still be some time, but I know down south they've begun. We use the flowers in all different ways -- elderflower fritters (fried flowers in batter, with the stems like a stick for fair food!), elderflower liqueur and wine, elderflower syrup and elderflower soda. I have a book on foraging and using elderflowers and elderberries (Elderberries: The Beginner's Guide to Foraging, Preserving and Using Elderberries for Health Remedies, Recipes, Drinks & More ) (aff link) with more information on spotting them and using them.
Below is an image of what the shrubs look like in flower. Once you know to look for them, you'll start to see them all over. One easy way to use the flowers medicinally is simply to dry them and use them in tea. Elderflower tea is great for colds and seasonal allergies. Just steep about a TBS in boiling water, covered, for about 5-10 minutes. As with all herbal remedies, a little often is better than a lot at once, so sip a cup of elderflower tea several times a day until symptoms improve.