From Kirbypedia

The extra game is a feature that opens upon completing Kirby's Dream Land. It can be accessed at the title screen by pressing Up + Select + A. It is the same exact game, except there are a few changes and the difficulty is significantly increased. In fact, if Kirby's Dream Land is too easy, the extra game will provide a challenge for even the more experienced player.

Enemies

The major difference is the replacement of some enemies for new and more challenging ones. For example, Shotzo is replaced with Blatzy which fires cannon balls much more rapidly. Enemies also vary in how much damage they can do, with damage ranging from 1-3 health.

• Cappy is replaced by Blopper
• Twizzy is replaced by Tookey
• Bronto Burt is replaced by Koozer
• Booler is replaced by Gaspar
• Shotzo is replaced by Blatzy
• Two Face is replaced by Mr. P. Umpkin
• Chuckie is replaced by Hurly
• Mumbies are replaced by Skuller
• Squishy is replaced by Flotzo
• Wizzer is replaced by Peezer
• Kabu is replaced by Grumples
• Dizzy is replaced by Benny
• Puffy is replaced by Cawcun
• Kookler is replaced by Whiskers
• Sir Kibble is replaced by Boomer

Difficulty

The difficulty is greatly increased from the normal game, and much of this is due to the new enemy moves, boss attacks, and overall speed and damage value caused.

The new and improved Blatzys fire very rapidly and can also rotate faster.

Glunk also fires much larger projectiles.

Spikes, found primarily in Float Islands, can take away 2 health bars if Kirby flies into or steps on one.

Parasols have become almost like dangerous boomerangs, in that when Waddle Dee or Waddle Doo ditches them to walk, they will fly at Kirby.

Bombs, Scarfy, and Coconuts all have a larger explosion radius. This makes them much deadlier opponents and items.

Gordos are the biggest threat, as although they still follow certain patterns, these are less predictable and often make them appear to be almost jumping around the screen. In addition, Gordos will take off a total of 3 health bars -- run into 2 Gordos and it is instant death.

Stars falling in the last area of Bubbly Clouds will do damage.

In addition, even the helpful secret room in Bubbly Clouds that is entered through the moon is just as dangerous as it is rewarding. While the Maxim Tomato and 1UP are falling, they will not appear at the bottom as you fall unless you fall with them at full speed. The catch, however, is that many of the deadly Gordos are falling as well and you can only really collect them as you glide in and out of Gordo range.

Bosses

While Bosses remain virtually the same, their difficulty is kicked up a notch, and some of their attacks vary.

Poppy Bros. Sr. moves and hurls bombs at a much faster rate, and in addition he dashes across the screen making him harder to dodge.

Whispy Woods drops regular apples, but he also drops Gordos. He also shoots out his air pellets twice as fast.

Lololo will not only shoot off blocks, but Gordos as well and he moves much faster.

Lololo and Lalala will move much faster.

Kracko chucks bombs, beams, and dashes around the screen much faster.

King Dedede moves twice as fast.