Issues in relation to which referendums are optional at the national level

Qatar
Constitution 2004
http://?www.?wipo.?int/?wipolex/?en/?text.?jsp?file_?id=225262
Article 75 The Emir shall seek public opinion on important issues pertaining to the interests of the State in a referendum. The subject of such referendum shall be deemed acceptable if acknowledged by the majority of voters; and the results of the referendum shall be binding and effective from the date of its announcement. The results shall be published in the official Gazette.
This does not include constitutional amendments.