na-adjective, noun which may take the genitive case particle 'no', adverb, noun
full of energy; brimming with vitality; very healthy
Describes a person or living thing that is visibly energetic, lively, and in good health. Often used in everyday speech to emphasize high spirits.
The children are running around the park full of energy.
今日は元気いっぱいだね。
You're brimming with energy today, aren't you?
元気 means 'healthy' or 'energetic' as a general state; 元気いっぱい emphasizes being completely filled with energy, like 'bursting with health'.
活発 means 'active' or 'lively' and focuses on behavior; 元気いっぱい focuses on the feeling of abundant health and vitality.
Standard spelling; いっぱい is usually written in kana.
Kanji spelling for いっぱい; less common but still seen.
Compound of 元気 (genki, 'health, energy') + いっぱい (ippai, 'full'). The phrase is straightforward and modern.