The sailboat retrospective is a model that I especially like to use at the end of significant chunks of work, like a release or the end of an epic or the end of a project. But it could be used at any time, especially if there is a need to better understand project objectives, risks, hindrances and helpers.
I like to use this model for post release retrospectives because it helps the team to focus on lessons learned around unexpected risks, the things that slowed the team down and celebrate the things had really helped us.
A simple retrospective exercise I like to use is the ‘one-word retrospective’ found in Getting Value out of Agile Retrospectives by Luis Gonçalves and Ben Linders. This exercise is particularly useful for helping teams deal with their feelings.
Simply ask every team member to summarize in one word how they felt about the last sprint.