At my grandfather’s funeral I wrote and read out a simple poem about my memories of him. It was a mixture of the serious and the lighthearted, and went down really well. But as others have said, don’t feel you have to write a poem, prose is fine - and it can be as short or as long (within reason!) as you want.
How about the Book of Ecclesiastes? That’s always a crowd-pleaser.
I saw that wisdom is better than folly,
just as light is better than darkness.
The wise have eyes in their heads,
while the fool walks in the darkness;
but I came to realize
that the same fate overtakes them both.
Then I said to myself,
“The fate of the fool will overtake me also.
What then do I gain by being wise?”
I said to myself,
“This too is meaningless.”
For the wise, like the fool, will not be long remembered;
the days have already come when both have been forgotten.
Like the fool, the wise too must die!*