US passport holder do not need landing visa for 90 days stay in HK?

A senior US citizen wants to enter HK for short stay for a week or so, she wants to know if she needs to apply any entry permit, and what's the fee for it. She also needs to renew her HK 回鄉證 during her short stay in HK. How fast the procedure could be? How many days she needs to save for her HK 回鄉證？
Thanks,