Hey, there are some good ideas here...kids usually find fun things to do in nearly everything. This is due to their fantastic imaginations.
I have done many things similar to those you have listed here like the beach and museums. My kids get bored very easy and their attention span in minus zero - yeah, I love 'em to bits.
My problem is getting my two youngsters off TV, this is not easy when crap like Ben 10 and Transformer come on. I am trying to get them to read more books to expand their knowledge into other areas. It is not easy being a parent. Hell no!