Replacing an ingredient with one that's functionally equivalent, or at least reasonably close, is a straightforward proposition. Replacing ingredients that bring a signature texture or flavor to a dish is another thing entirely. For example, if you're in the middle of a recipe and discover you have no nutmeg, your options are pretty limited. A few ingredients share nutmeg's flavor in varying degrees, but usually your best choice is to substitute a spice which, if not identical, is at least compatible.
Sibling Rivalry: Mace
Nutmeg's flavor profile is highly distinctive but not entirely unique. Another spice, called mace, offers a nearly identical flavor, and with good reason. Mace is harvested from the same plant as nutmeg, and in fact is made from the papery husk that surrounds the "nut" of whole nutmeg. Its flavor is a slightly sharper and more pungent version of nutmeg's, so you can use slightly less, but they're otherwise very comparable.
Get Into the Mix: Pumpkin and Apple Pie Spice Mixes
If you don't have any mace hiding in your spice cabinet either, it's worth making a second round of your shelves in search of spice mixtures. For example, any prepared pumpkin-pie spice mixture or the less common apple-pie spice mixture is likely to contain a lot of nutmeg. In fact pumpkin-pie spice often includes both mace and nutmeg, profiting from their subtle differences to create a more nuanced flavor. Sprinkle the mixture over eggnog or hot beverages in place of nutmeg, or incorporate it into your baked goods. Add the mix first, then stir in reduced quantities of your other spices until your nose tells you they've reached the right proportions.
Paying a Complement
When nutmeg's own flavor isn't an option, your best bet is often to find a flavor that will complement your recipe in its own right. Allspice is always a good choice, because of its chameleon-like ability to offer hints of other spices wherever it's used. In baked goods or beverages, two common uses of nutmeg, use your imagination. If your apple pie calls for cinnamon and nutmeg, add a pinch of ginger or clove instead. Their flavors aren't the same, but both complement the cinnamon. Similarly, if you ordinarily top your famous rum-spiked eggnog with a pinch of nutmeg, cinnamon or clove are suitable alternatives.
Passing the Bar
Aside from its role in your holiday eggnog, your home bar might offer a few less conventional flavoring alternatives. Spiced rum often has a strong nutmeg note and can be used in many recipes. Regular dark rum, as well as brandy and bourbon, have an aromatic character not unlike nutmeg's, and add pleasant flavors of their own. If your bar is unusually well stocked, you might even have a bottle of De la Grenade nutmeg-flavored liqueur, or the same company's nutmeg syrup.
If you're not looking for an emergency substitute but a purer, richer flavor, the best replacement of all for ground nutmeg is to buy the whole spice, and grate it fresh whenever it's needed. The flavors in nutmeg and other spices come from a range of perishable flavoring compounds, and some of them are so volatile they'll disappear in a matter of a few hours. If you grate your nutmeg as needed, you'll always get the benefit of those fresh flavor notes. Just remember that because it is more flavorful than the ground variety, you'll need to scale back the amount you use so it doesn't overwhelm your recipe.