Most hotels and hostels will let you leave your bags if you arrive before a room is ready. AirBnB (never used them) seems to be on a case by case basis as to when you might be able to get in - I know you can contact the owner directly and perhaps work out an early arrival.
Otherwise, many train and bus stations have paid lockers or "left luggage" storage you can pay for.
In a pinch, you might find other hotels or even restaurants that can store it for you for a few hours. Last month I stopped off the train in the town of Tomar, Portugal, which has no luggage storage. But I was able to leave my bags for a few hours at a hotel across from the station (they refused any money for leaving the bags). Another time, I got off at a small town along the Mosel in Germany to hike and was able to leave my bags at a local pub for a few hours. I'd be more comfortable doing this in a small town in a big city, though.
(Good for you packing light - it will make getting around sooooo much easier!)