I used to create a fact file on firms before my vacation schemes. I'd make sure I knew as much as I could about their practice area strengths/international strategy/training programme (I'd now add use of technology to that list). I'd also create three lists of questions, one directed at trainees, one for associates/partners and one for graduate recruitment. Once I received our timetable for the week, I'd also update that list based on any talks we had e.g., a talk from a particular practice area.
You may also want to try get a basic idea of what dispute resolution lawyers do:
https://www.lawcareers.net/Solicitors/SolicitorPracticeAreas/Dispute-resolution
https://www.chambersstudent.co.uk/practice-areas/litigation-and-dispute-resolution
It's something you wouldn't be expected to know already, but it can help provide you with the background context going in (so you can pick up things quite quickly). You can also use the firm's website to investigate who the important people are in your departments and familiarise yourself with the kind of work the firm has done in DR beforehand.
I also used to try mentally prepare myself before schemes. I know this is work experience, but the fact that you'll be assessed for a TC can often make it equally stressful. I don't have any specific recommendations for that as I think it's quite personal, but just remember, the fact that you were chosen for work experience means they already believe you have great potential.
In terms of impressing: be proactive and interested in the people/their work, be authentic (candidates sometimes shoot themselves in the foot a little by being OTT), treat every piece of work as if it's really important, even if it's just proofreading (although be mindful of any deadlines).
Good luck!
