This is a great summer cocktail, but goes down easy anytime.
I call it a Pineapple Boxcar, but it really isn't a boxcar at all. It's more like a Pineapple Margarita with no sour mix, but if you happen to order it out and call it a margarita, they will invariably put sour mix in it. Better to call it a boxcar and have them go, "huh?", than to call it a margarita and have sour mix drown out the pineapple.
Good white/silver tequila (Patron or Silver Jose are both good, but use whatever you prefer)
Grand Marnier (or Triple-Sec, or Cointreau)
Maraschino Cherry, w/ stem (garnish)
Fill a pint glass with ice, add 2 parts tequila, 1-1/2 parts liqueur, 2 part pineapple and 1/2 part Grenadine. Shake or stir. Garnish w/ cherry.
Vary the ingredients to your taste. The Grenadine addition can be anywhere from a pale pink grapefruit color (making it less sweet) to full red (much sweeter); I like it somewhere in between.