You will find many seasonal  ideas for parties, centerpieces, and decorations in my book Entertaining for Eternity,  The main thing is to keep them simple.  Candles, curls of colorful ribbon, shamrocks, hearts, Christmas balls, or fall leaves tossed about work well.  Stock up ahead of time by purchasing clearance items after a holiday or season has passed.  Then store them in clear plastic containers where they can be easily found next year and you’ll always be ready.