Permanent to let a creature tap for two mana?
A few cards do allow creatures to be tapped for 2 mana, each with its own restrictions:

Grand Architect has the ability &quot;Tap an untapped blue creature you control: Add {C}{C}. Spend this mana only to cast artifact spells or activate abilities of artifacts.&quot;

Joraga Treespeaker has the ability &quot;Elves you control have '{T}: Add {G}{G}.'&quot; as long as it has at least 5 level counters.

Sachi, Daughter of Seshiro has the ability &quot;Shamans you control have '{T}: Add {G}{G}.'&quot;