What do you do with a hole in the middle of the room?

When we had the new heating/cooling system put in last summer Botts and Tye (the company that did the work) also cut a new cold air return in the floor. The old return was then removed leaving a 24" x 25.5" hole in the middle of the floor between the drawing room and the dinning room (I don't have any pictures of the hole). The question then was what to do with an opening into the basement. We came up with several ideas such as:
1- use it as a trap door
2- put in a dumb waiter
3- cover it with plexiglass and then claim that it was where the money in the house was hidden (which by the way there was no money hidden in the house)
4- build a large fish aquarium in the basement underneath the hole, fill the aquarium with water and fish of course, and then cover the hole with plexiglass so that we could watch the fish from upstairs. (My favorite choice)
But logic prevailed so we decided to plug the hole by replacing it with some sort of floor design. We also wanted to use the old ceiling grate somehow.
(This is a picture of the opening with a new subfloor in place.)

This is the design that Chris came up with and so now the hole is no more.

Now we need to finish the floors in the two rooms.