Distribute blank index cards and tell the team they have 10 minutes to write a postcard to a person the whole team knows (i.e. an ex-colleague). When the time is up, collect and shuffle the cards before re-distributing them. Team members take turns to read out loud the postcards they got.
Set up a 'retrospective mailbox' at the beginning of the iteration. Whenever something significant happens or someone has an idea for improvement, they write it down and 'post' it. (Alternatively the 'mailbox' can be a visible place. This can spark discussion during the iteration.) Go through the notes and discuss them. A mailbox is great for long iterations and forgetful teams.
Find the source of problems whose origins are hard to pinpoint and lead to endless discussion Source:
Write the problem you want to explore on a sticky note and put it in the middle of a whiteboard. Find out why that is a problem by repeatedly asking 'So what?'. Find out the root causes by repeatedly asking 'Why (does this happen)?' Document your findings by writing more stickies and showing causal relations with arrows. Each sticky can have more than one reason and more than one consequence Vicious circles are usually good starting points for actions. If you can break their bad influence, you can gain a lot.
Let team members appreciate each other and end positively Source:
Agile Retrospectives who took it from 'The Satir Model: Family Therapy and Beyond'
Start by giving a sincere appreciation of one of the participants. It can be anything they contributed: help to the team or you, a solved problem, ...Then invite others and wait for someone to work up the nerve. Close, when no one has talked for a minute.