A tank as boo said is much preferred. A bowl for a fish is cruel. While it is not possible to entertain goldfish you can ensure that they are in the best environment you can provided within reason. All fish will be much happier when there are plants in the tank, this gives them a sense of security. Unfortunately, goldfish are quite fond of plants and unless there is a lot of well-growing plants, they may eat them all. In that case the plastic plants are advisable. If they are in a well lit position [although not in the direct sun] the fish will further benefit from having the sides of the tank covered with dark paper - perhaps decorated with a well meaning home-artist's impression of plants? - as well as the back ofcourse.
The ping pong ball myth evolved because goldfish will eat a lot more than is good for them if you let them. They are used to food being thrown in the tank from the top and much of it floats, as does a ping pong ball. They are really not playing with the balls, they are trying to eat them and as the balls are far too big for them it looks like the fish are playing netball.
You'll find that when the fish are happy they will be continually swimming around slowly looking around looking for food, it is in their nature. If they swim up and down with their nose against the glass they are not relaxed. Most of the gravel will be sucked up, chewed for any edible substance, and spat out again. If you can feed them 3 times per day with a minute amount of food they will live a lot longer, although once per day in the summer is sufficient, less in winter. When they are big enough give them an occasional treat and get a little bit of live bait from a petshop.