Add a random event when an adventure or quest is finished
Reveal a hidden image, item or challenge when quest or adventure are solved to keep things mysterious.
Maybe the adventure is not really solved before... a last task is taken on?!
While this is an obviously good idea from a motivation perspective, adding a task of unknown effort (esp. at the end) would break the sprint commitment of the team. We should be carfeful here.
I see this could be dangerous, ok. Do you have a better idea? I as a player would want a surprise element, it is really motivating.
Any kind of suprise that does not require an unkown effort would be fine with the scrum spirit, i.e. items or story elements would be fine. With a twisted arm: Maybe optional side quests could appear unexpectedly. If they are optional, they do not break the sprint commitment (and should only be considered, if the workload is appropriate). ;)