We have made a sunflower castle with my daughter for the past 4 years and it is such a great activity.  Buy a selection of sunflower seed including (from biggest to smallest) Mammoth, Giant, Hopi black, endurance and sunspot.  Dig out a circle in your yard with a 6-8 foot circumference and about 3 inches deep (ours has grown over the years to about 12ft).  Cover a circle in the middle about 1-2 feet circumference and a short walkway with mulch so no flowers grow there and there is a path to walk into the castle.   Then, plant the Mammoth seed around the middle mulched circle.  Measure 2 inches out and plant the giant seeds in a circle around the first set.  Keep measuring 2 inches out and planting the seeds from biggest to smallest.  When they grow up, the large ones in the middle will bow their heads in and make the "top" of the castle while the smaller one will grow up and make the "walls".  The kids love going inside and sitting in the mulched circle in the center.  My daughter has seen fairies living in her castle and it is host to many tea parties for the princesses.  Sunflowers are very hardy and we have had success inspite of hail storms, strong wind which broke stems and extreme heat.  While it is best to start the seeds at the end of April so they bloom by July, we started last years in late May and had a fall sunflower castle which lasted till October!