I've got an interesting one... I have a use case where an Excel file is created from a Java application. We want to protect the sheet using a password, but allow functionality such as sorting, autofiltering and deleting rows.
I've gotten as far as protecting certain columns and allowing edit on other columns using the POI library. However, allowing the other functionality is not possible with POI. Macros are out, as the spreadsheet will be e-mailed and macro's are typically blocked by most anti-virus or mail filtering programs.