List is probably the minimal intro you can do without trapping yourself. It's too easy to implement something that can do the 1-or-0 elements in Maybe, but still isn't a Monad. You should also do a conditional, which I believe this code fails; that is, a monadic computation can examine the intermediate results and make flow-control decisions.

The minimal example is probably something like:

    minimal :: Int -> [(Int, Int)]
    minimal i = do
        x <- [i, i + 1]
        y <- [i, 10 * i]
        if i `mod` 2 == 0
            then return (x, y)
            else return (y, x)

Running this in GHCi, I get:

    > minimal 2
    [(2,2),(2,20),(3,2),(3,20)]
    > minimal 3
    [(3,3),(30,3),(3,4),(30,4)]

The evenness or oddness of the parameter controls which side of the tuple gets which element.

It may still be possible to implement this without implementing a full monadic computation, but if you can pass this test using something like bind or join, you're probably at least on the right track.