Preston Garvey, Minuteman

At the beginning of combat on your turn, create a green Aura enchantment token named Settlement attached to up to one target land you control. It has enchant land and "Enchanted land has '{T}: Add one mana of any color.'"
Whenever Preston Garvey attacks, untap each enchanted permanent you control.

Horizon Stone

If you would lose unspent mana, that mana becomes colorless instead.

Fangorn, Tree Shepherd

Treefolk you control have vigilance.
Whenever one or more Treefolk you control attack, add twice that much {G}.
You don't lose unspent green mana as steps and phases end.

Upwelling

Players don't lose unspent mana as steps and phases end.

Doubling Cube

{3}, {T}: Double the amount of each type of unspent mana you have.

Forest

Temple of the False God

{T}: Add {C}{C}. Activate only if you control five or more lands.

Omnath, Locus of Mana

You don't lose unspent green mana as steps and phases end.
Omnath gets +1/+1 for each unspent green mana you have.