This is based on Mark 4:26-32. It would make a good sending or response prayer if you are preaching on that passage, or generally encouraging people to take a step of faith despite feelings of weakness and insignificance. It could also potentially work at Harvest time, or for an outdoor service where you can go and find seeds in a woodland.

Hand out a small seed to everyone - it could be a mustard seed or something slightly larger.
If you haven't already done so in the service, read Mark 4:26-32.
Read the following prayer over the congregation:

God, we hold these seeds in our hand.
We see smallness, weakness, insignificance.
You see potential, growth, significance.

Tiny seeds grow by your power into huge trees.
They provide food for the hungry and homes for the birds.
From the darkness of the earth springs forth your life, your harvest.

Your Kingdom is like a tiny seed.
When we look at your church, and our own lives,
we see smallness, weakness, insignificance.
You see potential, growth, significance.

Give us eyes of faith and willing hands to see your Kingdom grow in us.
May we feed the hungry with food and the bread of life,
welcome the homeless into your family,
and gather the harvest you love has provided.

In Christ's name and the power of the Spirit
we commit our lives to you again.
Amen.