Beaches, Sand and Muktzah

The Mishna Brurah says that something that isn't a kli or food or animal food is considered מוקצה מחמת גופו and he includes dirt, sand, and stones. Can you walk on the sand? Is a sandbox muktzah on Shabbos; can you designate the sand for use on Shabbos? Can a toy which is not a kli (e.g. sand) become non-muktzah via designation?