I found a good nonverbal task for retention intervals. It is a puzzle called picross.
The goal of the puzzle is to complete a picture by filling out boxes in a matrix. Numbers in the columns and rows of the matrix indicate the numbers of boxes that have to be filled out. Each number indicates how many consecutive filled boxes there are on a row or column. The combination of the numbers in the row and columns lead to one solution that results in a picture.
This puzzle is a perfect alternative to Sudoku and can easily be employed in experiments.
Some sample puzzles can be found here: Picross Puzzle
Have fun!