Your fresh flower mix may contain different flowers than the ones I've selected for my pumpkin centerpiece. You need to examine your flowers before buying to ascertain if they are fresh.
The first is a sunflower. The bright orange red is a delightful focal point for any autumn centerpiece. The petals should still point upwards (they may flatten out and down as they draw water) and the stem should feel solid and look green (not yellow).
Asiatic lilies should still be closed, maybe cracked up slightly to show a bit of color. These blooms will open up in the next day or two as they are resting in your centerpiece. The leaves should be glossy green with no yellowing or drooping.
Alstromeria (also known as the Peruvian Lily) is a great fall flower and is available in bright oranges and yellows. The flowers should still be in bud form, ready to open as they draw water up. The leaves should be green with no signs of yellowing.