These are recommended plugins. If you find a better one let me know.
Drawing - Gliffy
Prototyping - Balsamiq
Spreadsheet Functionality in Tables
Comparison chart with key features:
|Evaluation Worth Using?||Plugin||Math Formulas||Inline Editing||Works in Mobile||Filtering||Comment|
|To Test||CelesteCS Math||Looks promising...|
|No||Bob Swift Advanced Tables|
Add only is not enough.
|April 2015 - Like the most but requires math functionality.|
|No||Excellentable||April 2015 - Nice search function but ugly.|
|No||Spreadsheets for Confluence||April 2015 - lack of InLine Editing made it not good enough.|
|To Test||Table Filter and Charts|
So far all these are pretty limiting.
Yes this generally works but sharing spreadsheets for editing requires sharing in Google and has some limitations (forget now), but not for Corporate Websites.
Securing Specific Content on a Page
Redaction - looks very promising.
Looks promising, no public key, but simple enough - https://marketplace.atlassian.com/plugins/com.prcpo.confluence.plugins.secure-content-block
http://confluence.atlassian.com/display/CONFHOST/Capturing+data+with+forms - seems to have a good run down of the available options.
http://blogs.atlassian.com/2009/05/confluence_wufo/ - expensive but looks to be very powerful and mature. Video demo on home page. Requires an additional server.
Still looking, have not tried anything yet.