Here are a few fun essentials to building your in-home bar.
Glassware. These can come in any shape and size! Champagne flutes, martini glasses, wine stems, rock glasses, etc.
Fresh Garnishes: Every cocktail tastes a little bit better with a fresh cut lemon/lime or a mint leave. It's also a killer way to pop in some color and add some additional flavors.
The Booze. It's good to have on hand: Vodka, Rum, Tequila and Gin. This way you have a little bit for everyone, and you can create a variety of different cocktails.
Straws. These can serve as two things.
1.) A cocktail stirrer
2.) A decor piece, you can mix in any theme you have going.
Mix It. Its nice to have a variety of low-sugar mixers. Some good choices are: club soda, champagne or sparkling, and ginger ale. You can add a splash of ginger ale or some bubbles to sweeten up the cocktail.
One of my favorites...
4 oz light rum, 1 tsp of lime juice, 6 fresh mint leave, 1 tsp of cane sugar, and club soda.
Muddle the lime, run, sugar and mint. Add the club soda, and boom you have a delicious summery beverage.