There is no limit to what you can concentrate upon when you meditate. A meditation is merely emptying your mind of everyday thoughts, and so you are allowing your subconcious mind and your higher self to speak, you are unlocking doors to your childhood memories and even past lives, after a while even humanity or the voice of God.
Some people simply meditate upon Nothing, Emptiness, Space, others have a mantra which they chant. You can also meditate upon pattern, spirits, nature phenomenon, the simpler the object the easier the concentration, and to come to a relaxed and inspired state of mind.
Yesterday, after christmas dinner, I went for a walk under the nearly-full moon and i meditated upon snow.
Snow is a wonderful carpet under which the earth can gather strength to achieve new growth in the spring. Like us, the souls, all snow crystals are unique, but equal. One snowflake seems not important, yet it is one of natures most beautiful masterpieces. Together the snowflakes makes a mighty mass that can give blessings or disasters.
One warm breath and the snowflake is gone - as is the human body. But it doesn't disappear --- it merely transforms into a drop of life-giving water, which is it's true nature. So do not fear death, you are simply part og the cycle. Next winter, you can fall as another snowflake - if you so choose.