There are a lot of different options for retrospectives depending on what you want to get out of them. The book Agile Retrospectives by Diana Larsen and Esther Darby is a great place to look.
I also highly recommend looking at Retromat for ideas.
As a final piece of advice, I would consider using the 5-stage retrospective talked about in both of those two resources. While it may feel a little over-structured and take a little longer, it ensures that the format of the retro is solidly geared towards valuable outcomes.