Madcap things are done in a foolish, rash, or impulsive way, without planning or thought. Your madcap adventure might include a spur-of-the-moment bus ride to a nearby city without enough money in your pocket to buy lunch.
The adjective madcap, which can simply mean "impetuous," can also have a sense of "funny and eccentric." A movie that's described as a madcap comedy will probably be full of broad slapstick and laugh-out-loud physical humor. Madcap, dating from the 16th century, originally meant "lunatic" or "crazy person," from the "head" sense of cap — in other words, a madcap was a "crazy head."