I use a recipe similar to Dan Lepard's for focaccia.

• 100% flour (obviously)
• 35% sour starter (100% hydration)
• 0.74% yeast
• 2.5% salt
• 65% water
• 5% extra virgin olive oil 

When it cools, it's extremely light and fluffy, with HUGE holes in it. The closest you'll ever get to eating clouds. But for some reason, it goes tough and hard within 24h despite being kept in an airtight plastic box.

WTF?! Should I store it differently?

When I make 'dry' bread (i.e. without oil) it stays nice for days on end... sometimes up to a week.  But this is no doubt because it's already dry, so it doesn't matter so much that it's getting drier every day...