It doesn't have to be is complicated. Sure, you go all out and bake for days, but you can have a more relaxed tea/coffee with store bought items. Here are some things I do to prepare for a tea party on the fly.
I keep a box of nice tea in the pantry. You can get boxes of assorted teas from Celestial Seasonings that I think are quite nice. I recently have tried the Green Tea assortment from Mighty Leaf and enjoyed it.
I serve half and half but most people are happy with a powdered creamer.
I also keep an assortment of pretty paper napkins for the season. I pick up a few packs at the beginning of the season to be ready for an impromptu visit.
Don't forget about temperature. If it is a warmer season, turn down your a/c a few degrees. Playing hostess can turn your internal temperature up a bit so keep yourself comfy.
Turn on some nice music and light a candle. Keep your scented candles away from the table so the smells don't compete with what you are serving.
Always keep a tablecloth on your table and/or a seasonal decoration.