It's NICE to be a person trapped inside George Michael's stache?
inside a thing, OR inside of a certain time frame(eat it while it's hot! 温かい内に食べて!) Also, it's a slang term for 'us.'
the contents (of a box or a book or whatever)
a guide - usually in the form of a map, or an information kiosk. Also if a clerk shows you around a store pointing out things you might want to buy, that 's annai.
within xxx (as in, "I'll be there within 10 minutes!")
usually defined as 'confidential,' but that makes it sound more serious than it is - like a spy document or something. Actually, 内緒 is kind of a kids' word. "Keep this information between us two."