Whether it be drinks in Manhattan or barbeque in the backyard, choose an engagement party theme and style that you know the happy couple will love.

Cocktail Party
The best thing about cocktail parties is they are much less work than a dinner party, and usually a lot more fun if you're going to have more than 15 people. For a memorable cocktail party, play some great music to keep the party lively, make sure you will not run out of drinks before the end of the night, and choose to serve liquor over beer or wine. You could even make up a drink and name it for the newly betrothed couple. Remember: offer some delicious virgin concoctions for those who don't drink, and keep the appetizers simple and hearty (to absorb the booze).

Outdoor Picnic
This is a romantic notion to celebrate an upcoming wedding. Find a beautiful spot in a local park or garden, ask 20-30 of your closest friends and relatives to show up at sundown, spread blankets out, watch the sunset, and make a toast to the newfound joy of your friends. Serve simple fruit and crudites, and break out the wine for a lovely and romantic summer evening.

Theme Party
If you know of something particular that the couple loves, center the party around that and make it your theme. Scrabble, Elvis, ballroom dancing, Las Vegas, bowling, the great outdoors, and Hollywood celebrities - gotcha thinkin'? All of these are themes that would be easy to replicate as a party theme or activity.

Dance Party
If your group loves to get down, why not have a dancing engagement party? You could choose a style (swing, ballroom, two-step, disco) and move back the furniture. Ask the groom's brother or best friend to be the DJ and narrate the evening with jokes and funny stories about the couple. To get guests to mingle, do the ol' switcheroo - during the middle of the song, ask everyone to switch partners.

Honeymoon Destination
Does the couple have any idea where they want to go on their honeymoon? If you have an inkling of this privileged information you could throw them a Hawaiian luau, a Tuscan inn party, or an Austaralian Outback soiree. Serve native food, and ask everyone to come in costume.