Place a table on the lawn, and lay a tablecloth across it. The tablecloth can be red and green for a Christmas party, black and orange for Halloween or bright and festive colors for a birthday.
Attach streamers across the top of any fence or railing in the yard. Secure them to the fence or railing firmly with either tacks or tape to prevent them from blowing away in the wind.
Tie balloons to any railings in the yard. Doorknobs and chair backings are other places where balloons attach with minimal effort.
Tape a party sign to the front door. The sign can read "Happy Birthday," "Merry Christmas" or include a personalized message for the specific party.
Rent a bounce house from a party store, and place it in the lawn if your party will have kids. The bounce house will keep kids occupied while also providing a bright-colored decoration for the celebration.
Make or order hors d'oeuvres that match the color scheme of the tablecloths and other decorations. The hors d'oeuvres can then function as decorations themselves.
Light candles as it gets dark. They'll add to the overall atmosphere of the party while also providing lighting. Place one on each table, and keep it in a candle holder that prevents the candle from blowing over.