Here's a drink from David Embury's The Fine Art of Mixing Drinks. The section on Rum Sours is prefaced with his statement that he considers the Daiquiri the most outstanding of the lot. I wouldn't disagree with that sentiment (or pretty much anything he says. He may be opinionated but he's usually spot on). He then goes on to list a handful of variations including the Beachcomber, Airmail, Knickerbocker and one which he created, and named after his favorite community, the Larchmont.
White rum, lime and sugar are all present but this one veers away from the Daiquiri by augmenting the sugar with Grand Marnier or, as he calls it, "the king of liqueurs". The inclusion of Grand Marnier here works and takes what is usually a crisp, bracing and refreshing affair and rounds things out a bit. The texture itself is smoother and there's a pleasant, slightly mysterious depth imparted by the sweetened brandy and orange liqueur.
Larchmont* - David Embury, The Fine Art of Mixing Drinks, 1948
2 oz White Rum
3/4 oz Grand Marnier
3/4 oz Lime Juice
1 tsp Simple Syrup (Embury's is 3:1**)
Garnish - Orange Peel, optional
*Embury's recipe is actually listed in parts (6:2:2:1/2) and the amounts specified above are an imperfect approximation of those parts. If you've been looking for an excuse to buy metric jiggers this type of recipe converts easily to milliliters (30 ml is about an ounce). Then it looks like this:
60 ml White Rum
20 ml Grand Marnier
20 ml Lime Juice
5 ml Sugar Syrup (5 ml is about 1 tsp)
**Thanks John, for reminding me it's 3:1. I had forgotten this important tidbit.
Random - Embury considers the "original and correct" Daiquiri to be 2 oz white rum, juice of half a lime, 1/2 tsp sugar. Cited among its attributes - "[t]he reaction time is short." Earlier in the book, while discussing his 3:1 simple syrup he bemoans the wasted time spent dissolving sugar in drinks like the Daiquiri and eventually details the 8:2:1 ratio (using simple syrup) for which he is well known. Its not quite as dry, but it's still bracing and plenty delicious. Difford's 10:3:2 ratio (using 2:1 simple) is another popular version and one which I turn to often.