For each x minutes offline*, apply an additional y% for (z*x) EXP.
* and maybe when resting with a campfire nearby, to encourage engaging with others players and strengthening the community feeling.
That is, apply a cumilative bonus (with a maximum) EXP bonus to encourage resting and letting more casual players (less available time, other reasons).
This can be seen in other popular MMORPGs (read, World of Warcraft, and clones/similar games), and doesn’t make a massive difference, but after a long break, you can notice time better spent in regards of exp/time ratio, which helps casual players.