Here are a couple we use...
For Morning Meeting (to the tune of "Are You Sleeping):
Morning Meeting, Morning Meeting
On the rug! On the rug!
Everybody come, everybody come
To the rug, to the rug.
I got it out of a Scholastic book on Morning Meetings, just to give credit where credit is due.
Writer's Workshop (also to the tune of "Are You Sleeping"):
Writer's Workshop, Writer's Workshop
Come and see, come and see!
Look what we've been writing
Isn't it exciting!
Come and see, come and see.
Here's a link to an old post about walking in line songs/rhymes...