Looking for a gift idea for someone who loves bar-tending but already has great glassware and a bar tools set.
How about some home made infused simple syrups or vodka (i've done chinese 5 spice vodka, coconut-ginger-lemongrass vodka, blueberry lavender vodka, and orange vanilla vodka as gifts). Or some fancy liqueur like Domaine de Canton or St. Germaine. Or some fancy bitters http://www.atthemeadow.com/shop/Drink/Bitters/Bittermens-Bitters
bitters and bar syrups like the ones available here. so much fun to play with!
An assortment of bitters would be cool (Fee Bros. has a bunch of different kinds including this one in our shop: http://bit.ly/i4su4m), as would be high quality garnishes like maraschino cherries, olives and onions. For a friend, I put together the Ideal Manhattan kit, and bought everything he needed to create top notch Manhattans. Not cheap, but he was thrilled.
I love collecting vintage cookbooks and there are some great vintage bartending/cocktail books available. Check out Bonnie Slotnick http://bonnieslotnickcookbooks.com/, or alibris.com. Could go along with some of the great suggestions above.
A friend of mine has been perfecting his gin and tonics by trying different gins and fancy little bottles of quinine. You could put together a G&T kit like the Manhattan kit suggested, or some other drink your friend likes...
A set of pretty liquer or cordial glasses is always nice. And there are tons of great shakers out there -- a good friend has one that looks much like a teapot that I covet!
Maybe a more unusual drink kit, if your friend already has the makings of manhattans and g&ts--I gave a friend a New Orleans Sazerac kit, and he loved it: a nice bottle of rye, herbsaint, and Peychaud's bitters.
If you are thinking about doing a drink kit, the Aviation Cocktail is one of my personal favorites. It's got gin (I prefer Aviation), maraschino liqueur, creme de violette, and lemon juice).
"Artisinal" (lord, I hate that word....) olives and cherries for the "drop-ins" in classic cocktails (and no, I don't mean the "blue" [sic] cheese stuffed olives you find at the megamart or warehouse store. There is sort of a cult around these things. Google "Luxardo" for cherries....
Cool cocktail picks for spearing said artisinal olives and cherries. I have some that are glass, and look like an olive (green, replete with a red pimento inside) fused to the top of a clear spear. I have others that are plastic, and look like miniature Martini glasses perched on top of a clear plastic spear, again, there's a teeeeeeny-tiny olive, spear and pimento in the little glass. Way cool.
Cool swizzle sticks (those are easy to find, look at Cost Plus/World Market or Pier One) for mixing things like Cuba Libres or Gin and Tonics.
Cool tonic waters like Fever Tree or Q Tonic. Way better than what you can get in the megamarts.
Cool liqueurs like Luxardo Maraschino, Cynar, Creme de Violette, Campari, Benadictine, St. Germaine, Pernod and so on.
The possibilities are endless. If you have time, look at some cocktalian forums. Just Google "Drinkboy", "Cocktail Database" "eGullet" (once there, search for cocktails) and "Esquire Drinks". They will give you limitless options.