Hey!
Many open days have workshops in which GR goes through how to make strong applications for them, so if your upcoming open day has that then those can be super useful. Some firms even take the time to give you personalised feedback on your open day application and so you would be able to use that information to craft a stronger one for the vacation scheme.
Most importably though, you should use open days to ask any questions that may be helpful with writing your application. Before the open day, I would already strongly recommend that you have a draft version of your "Why X firm?" question and that you already do extensive research into the firm. At the open day, you can ask targeted questions that relate to your reasons for wanting to work at the firm. This will fill in some of the things you didn't find online and give you more material to reflect on. The added benefit is that by asking informed and thoughtful questions, you might be able to stand out to GR.
Also, just in general, it's a good practice to have questions prepared even about aspects of the firm that you might not be writing about. Having lots of questions prepared will also be useful for when you do interviews at the firm. And be sure to note down any new questions you have as the open day goes on, and ask them (if appropriate) or keep them for later.
You can also use the open day as a way to build contacts at the firm that you can later ask additional questions from. Lawyers are super busy so not everyone would be happy to receive unsolicited messages. But sometimes people explicitly tell you that you may contact them with further questions.
Finally, many open days have skills sessions, like negotiation exercises and interactive workshops on practice areas. Make sure you note down what you learnt and how you felt when partaking in these. These will be important for when you describe what you did on open days on applications and also give you a chance to reflect on whether commercial law is for you. (Also, generally just take good notes throughout and then type up a write up soon after the event so that you understand your notes!)
Thank you for such a detailed response. I will definitely make notes and ask lots of targeted questions!