Waku waku (わくわく) is the feeling of happy excitement about something that is coming: “I can't wait!”
The suitcase is packed, the alarm is set, and you cannot sleep because tomorrow is the trip. That bubbling, can't-wait feeling is わくわく.
What it means
わくわく is excitement about something good that is coming: a trip, a concert, a new game, the first day of something. There is no fear in it and no worry. It is the feeling of looking forward, close to the English “I'm thrilled” or “I can't wait”.
How to use it
Add します, the same as どきどき: わくわくします (“I'm excited”), わくわくしています (“I'm excited right now”), わくわくしました (“I was excited”). It is almost always about your own feeling, and almost always about the future.
あした、にほんへ いきます。 わくわくします！
I'm going to Japan tomorrow. I can't wait!
いいですね。 なにを しますか。
How nice. What will you do?
ラーメンを たべます！
Eat ramen!
Close cousins
どきどき has nerves in it: the heart pounds because something might go wrong. うきうき is a light, cheerful mood that shows in the way you walk: the first warm day of spring. そわそわ is restless waiting: you keep checking the clock and cannot sit still. わくわく is the purely happy one.
In manga and anime
Look for ワクワク in katakana beside a character with shining eyes and clenched fists, usually just before an adventure begins. Small children say it out loud, which is why it often belongs to the youngest character in the story.
Can you use it?
Yes. Adults say わくわくします about holidays and new projects, and it sounds warm and honest. In a formal email, use たのしみに しています (“I am looking forward to it”) instead.