We found that a lot of people were customizing the IKEA Lack table, which is a square side-table that is quite cheap (a white one is $7.99). Here are some examples of what people have done.
We decided to hack a Lack too and showcase a 500 piece circular puzzle that we had made. The Lack table-top is a 55cmx55cm square, and the diameter of our puzzle was almost a perfect fit. The puzzle was set and "frozen" together with Modgepodge (which you can get at any art store) and pasted to a cardboard piece. We then glued the cardboard back of the puzzle to the top of the table.
Next, we wanted a flat table top that is transparent. We bought a sheet of acrylic from Home Depot, and cut it to size. Now ideally you should use some tools to do this (the best one is a laser cutter). But we did it manually, and it took forever (about 20 minutes of scraping for every cut you make, followed by a careful snapping along the cut). The acrylic was kept level with four carpet savers.
Finally, we added some decoration with paint. Since the puzzle was a set of scenes from Romeo and Juliet, we added a quote from Shakespeare along the table edge ("What's in a name? That which we call a rose by any other name would smell as sweet."). We also drew some red roses to go with the theme. Just think of it as our way of sticking it to the crazies who believe in Anonymous: