It's easy to learn the basics of Kirby's Adventure in the relatively tame
Vegetable Valley. The boss of this introductory level, Wispy Woods[sic], should be familiar to you from Kirby's earlier adventures!”
The intro to Vegetable Valley changes slightly from Kirby's Adventure to its remake, Kirby: Nightmare in Dream Land, but both are generally the same; both involve Kirby having a conflict with a Blade Knight in a wooded area.
In the original game, Kirby is having a showdown with a Blade Knight. Kirby parries most of his attacks but his blade gets knocked away. Kirby inhales and eats Blade Knight for the Sword ability before he gets pushed off the platform.
In Kirby: Nightmare in Dream Land, Blade Knight is advancing towards Kirby on a large wooden platform. Just as Kirby is about to fall down, he swiftly inhales and eats him to get the Sword ability. Kirby also encounters Blade Knight at the Museum, which may be a reference to this intro.
Vegetable Valley consists of only 4 normal stages, each of which are rather short. Vegetable Valley is a rather simplistic level that takes place predominantly inside a vast forest and up in the wooded treetops, although lakes and plains can be seen in the distance of some stages. Similar to many first levels, Vegetable Valley features beautiful grasslands and forests. However, Vegetable Valley, being a valley, also contains caverns (and some mountains) as part of its setting. Most of the platforms in Vegetable Valley, especially in the later stages, are floating tree branches. The level introduces some of Kirby's most basic abilities like Fire, Spark, and Sword. Whispy Woods makes his home in the level's forest, and acts as the game's first boss.
In the second stage, the player can view Star Blocks arranged in a fashion that spells out HAL. To do so, the player must enter the second stage and make his/her way near the Warp Star. By waiting enough time (and by going backwards to get the Warp Star out of view), the Warp Star disappears and Kirby can move forward to the area with the HAL logo spelled out. This was removed in the remake, but a similar thing could be done in Illusion Islands in Kirby Super Star Ultra.
This can also be done on the Virtual Console version of Kirby's Adventure as well as in the original NES version.
The UFO room
There is a hidden room with a UFO inside in the first stage. In the second area of 1-1, there is a series of four blocks at the first waterfall in the level - The first block is different from the rest. Have Kirby float underneath and press ↑ on the +Control Pad to reveal a hidden door, and he'll be taken into the room with a UFO.