Make a blanket fort.
Growing up, one of my favorite things to do with my younger brother was to make forts. Either out of stools/chairs and blankets, or sofa cushions and blankets. Then we would watch movies and eat popcorn while we laid inside, or we filled it with pillows and blankets and read books or told stories...or we even played house.
I think no matter what age you are, forts are fun for all. Again, bring out your creative side and have fun. It doesn't have to look perfect. You can tell the kids it's a castle and they are the king and queen and their imaginations will just run wild. That's what it's all about.