ok taking construction is soo sick! it's just a free class to go and build and chil! take advantage of the resources and the time, and build something creative without making it sketchy-style... you'll have everything you need to make a legit feature.
if I were you, i'd either make one of two things. You could make an existing but cool feature like an s-box or a rollercoaster box, or even an rollercoaster-s-box, and just completely nail it down to the last bolt and make it perfect. Or you could make an innovative and unique feature - like something that moves or something... a teeter-totter box was already suggested... you could build a sick banana box and put it up so that it kinda forms a J, and you ride up from the small side to the really tall side of the J, and then it brings you back down (understand? If you don't i'll draw it out for you. my park did it once and it was soooo fun!)
Anyway that's what I'd do...