Learning Japanese is going to be tough if you attempt to learn an entire dictionary's worth of vocabulary through memorization and repetition. Learning by rote is slow.
However, learning with fun methods, like games, can be a much better way to keep new vocabulary in your memory. I've found several games to learn Japanese with on the internet, but most are not terribly complicated.
You can go to
Digital Dialects for some basic vocabulary and kanji-meaning games, which may help break up whatever
other method you're using to study.
You can practice your かな (ひらがな and かたかな) [Kana (Hiragana and Katakana)] with
Real Kana.
Don't miss out on playing
Kana Space Invaders, however my game crashed the very moment I stopped the oncoming invasion.
Finally, if you think the game part is more important, and you like games reminiscent of old-school Final Fantasy, then download
Slime Forest Adventure.