I would'nt use helium outdoors unless I had to. If i can use air columns then I do. You can do air and use a 30" or larger helium balloon to help keep it up. Make sure that your column is packed tight to keep it's shape. You could use sand weight, flat heavy brick (cover with colorful wrap). And try to use light colors outside as they will oxidize pretty quickly unless you coat them with shine. if you use really light colors that's not such a problem.
