Are you taking any classical language(s)? If so it might be worth doing a little preliminary work on those over the summer (the department will probably tell you if you should though, and may be able to make recommendations otherwise; for ancient Greek at least it might be worth learning the alphabet since it's different). The Greek/Latin to GCSE books by John Taylor (et al) seem quite commonly used for both degree level study from scratch as well as preparation for GCSEs, so could be worth starting on (?) if you can find them in your local library or get a cheap copy, maybe. You could also/instead attend a summer school (e.g. the JACT one) in the/a language(s) if you are going to be taking one, possibly.
However unless the uni contacts you to provide some required (or recommended) reading or preparatory work for the summer period, it's really just anything you are interested in. You could read some of the commonly studied texts (literary or historical, or possibly philosophical if you wanted) in translation, to whet your appetite. If you can, visit some museums to see if any of the material culture from some specific period or region strikes you, and then use that as a starting point to look a bit further into that period (or region).
You don't have to restrict yourself to just the historical texts and you could look at the literature and/or philosophy of the period, and maybe secondary writing on the archaeology/art history/material & visual culture of the period/region to provide more cultural context for the history side of things (and vice versa!). Obviously though if you're more focused on the ancient history aspect then you may be less interested in the literary works (although these can provide context for historical reading).