I recently stumbled onto the wikipedia entry for “garden path sentences.” Here’s an example of a garden path sentence from that entry:
The old man the boat.

Here’s another:
The horse raced past the barn fell.

There’s more. Reading the above two at first sort made the inside of my head throb (it made my brain hurt).

I mentioned this to an old friend today and he said “oh, it’s mental push ups. Cool.” Then he recommended the following as another version of mental pushups, for people who know how to count to ten in two languages or more.

Count to ten alternating languages. For instance:
One, dos, drei, four, cinco, sechs, seven, ocho, neun, ten.

Neat. The next step, I’ve not tried it, would be to do basic math problems in a similar fashion.