Coin (common or rare) and Gems
Treasure (Art Works, Jewellery, Furniture/Fittings, Prize Horse...etc)
Monstrous Pet (Griffon, Pegasus...etc)
Land and Titles
Advance in a hierarchical Organisation, Society, Guild or Faction (renown) - perhaps requiring reallocation of post (i.e. Archbishop)
Acquisition of Vessels, Keeps, Caravans or Tavern/Inn, Smithy, Library ...etc
Reputation and Prestige
Languages and Tool Proficiencies
Faith (per DMG)
Contacts, Allies, Connections - i.e. Religious, Scholarly/Academic (Sages), Underworld, Royalty, Craftsmen, Navigators, Monsters (Dragons, Outer Realm Beings)
Romance (Courtship, Beating of Suitor/s, Engagement, Marriage, Children)
Spells
Lore i.e. Research Material, Rare/Archaic Maps, Planar, Teleportation Circles
Magical Item improvements i.e. A
Ring of Jumping that may provide limited flight, weapon imbued with an additional minor property
Acquisition of rare materials for spells or magical items
Plot Points (per DMG)
EDIT: @
Reynard despite the above, my advice for your game is not a glacial advancement of 1 RL year = 1 Level up, unless of course if your players really buy into that style of game.
Rather...
Cap Hit Points based on size;
Change up the Rest mechanic and tie it into the Exhaustion mechanic (I'm using this);
Buff up monsters (that goes without saying);
Require something extraordinary in the fiction to advance characters from level x to y.
(For instance, in my campaign they destroyed a Beholder that had been feeding on the essence of a dead deity, once they destroyed the beholder that essence was released and the PCs were awash with it, allowing them to access level 10 and higher);
Change up the setting from village to town to epic to cosmic. Do not be afraid to have the PCs be feared;