How do you say this in informal japanese?

Question:

How do you say "I don't know" or "I wonder.." in japanese?
I tried to look this up on Google, but I couldn't find the phrase/word I was looking for.

Like, if someone asks you "What are you going to do tonight?" and you don't know, so you say "Hm.. I wonder" or "I don't know". Not "shiranai/shirimasen", but another word.. It's kind of hard to explain O__o"

I'd like to know how to say this in an informal way, but not so rude that you can't use it in a daily conversation with friends and so on. If you know the formal way of saying it, please tell me that as well ^^

Thanks!

Answer:

It really depends on what kind of question they are asking you.

If they asked you "What are you going to do tonight?" and you don't know (which means you are just thinking about what you are going to do this evening right?) then Japanese people usually answer with such phrases as "Mada kimatte imasen" (= I haven't decided it yet) or "Dou shi yo kana" (same meaning, but sounds more natural and girly and informal)