I've tried drinks with tonic water idea before for a nice blacklight glow, but it just seems to add too much bitterness. Also, peach schnapps generally seems to go best with citrus-y flavors. For punch, try the schnapps mixed with ginger ale, orange juice, and some lime sherbet.
Here's one of my personal favorite punch recipes so far (based on
this recipe):
1 (6 ounce) can frozen lemonade concentrate, thawed
1 (6 ounce) can frozen orange juice concentrate
1 (750 ml) bottle Southern Comfort
1 (2 liter) bottle lemon-lime flavored carbonated beverage
6 fluid ounces lemon juice
sliced fruit (oranges & lemons) for garnish