One way, using macros:

Paste these in a VBA module in your workbook. Change the sheet names and
password as desired.

Sub HideSheets()
Sheets("Sheet2").Visible = xlSheetVeryHidden
Sheets("Sheet3").Visible = xlSheetVeryHidden
End Sub

Sub UnhideSheets()
Dim ret
Const Pwd = "tTyY7&uU"
ret = InputBox("Password to unhide sheets", "Unhide Sheets")
If ret = Pwd$ Then
   Sheets("Sheet2").Visible = xlSheetVisible
   Sheets("Sheet3").Visible = xlSheetVisible
End If
End Sub

These macros could be called from buttons on another sheet. You should also
password protect the VBAProject for the workbook. This will prevent most
users from seeing the sheets or the pasword, but the security in an Excel
workbook is far from unbreakable once the workbook is open.

Hope this helps,

Hutch