i don't know what to tell you about the bedding. i guess i would just go with white or some other coordinating solid color.
for the tent though . . . you can use a hula hoop to make a mosquito-netting-style canopy that attaches to the ceiling. if your green sheets have a slot at the top edge (where that thicker hem is, know what i mean?) you can just cut the hula hoop once and slide the sheet(s) onto it, like putting a curtain on a curtain rod. that way you don't have to cut or sew anything. it would be open at the top - i guess you could just tie on some kind of strap and hang it from a hook on the ceiling. oh, tape the hula hoop back together of course.
if you want to devote more time, you could sew this from any fabric that would like nice in your room. or, if you're willing to spend the money, you can make it with curtains that coordinate.