Buy a bar and bar stools that can withstand the elements even if your patio area is covered. Stock the bar with glassware and hors d'oeuvre plates. Consider using high-end plastic barware to limit breakage.
Place the bar close enough to an outdoor outlet so that you can install a mini-fridge under the bar. Consider placing the bar right next to the house so that you can use existing plumbing to install a small sink for washing up and making drinks.
Use umbrellas, sun sails or a retractable awning for protection against the elements, or place the bar area under a large tree. Grow shrubs to act as a windbreak.
Choose decorative patio lights, tiki torches or outdoor lights with dimmers for romantic evenings. Create the impression of an intimate room by only lighting the patio or deck.
Create a seating area with comfortable chairs and a table or use lounges and end tables. Place candles or hurricane lanterns on the tables. Buy furniture that can withstand the elements.
Purchase outdoor patio heaters or build a fireplace to extend the usability of the bar into the cooler seasons. If the bar area is on a covered patio install a ceiling fan for warm days and evenings.
Use some form of insect control. Buy a bug zapper or use citronella candles.Grow plants that provide natural pest protection.