It's been ages since I've ridden a jumper but have a couple ideas.
Bright indoor/outdoor carpet makes a good cover. Over barrels, it makes a great roll top, just use extra poles at each side of the barrels and tuck the carpet under. Old fake christmas tree parts in brush boxes with the lights. Bright colored streamers, bags, pool noodles, balloons, etc added to your standards. Sheet of plywood to make a brick wall, I made myself one as a teen and then my friends & I did grafiti on it after I'd painted the brick.