I've tried a few combinations but alcohol on ice cream doesn't work for me.
Julia Child has a recommendation in a 1970's book, one of her volumes chronicling a latter-day TV show where she featured American cuisine and other dishes for relatively informal parties. The dish is vanilla ice cream in a dish, sprinkle teaspoon of instant coffee on top, and pour a jigger of bourbon or rum over. That's it. Simplicity itself and an excellent taste.