Second the Cabin Max bags and looking for 'Easyjet specific' to maximise space. Although for future flexibility, you might want to check it will also meet Ryanair, Tui or Jet2 limits as well if you might use those airlines.
Just checking that you'll have enough capacity with one small bag?
You are aware that strictly, you're not allowed a separate handbag, but people do report being able to get away with a small one under their clothes, eg the Uniqlo shoulder bag that was popular a year or so ago.
Before buying a bag, think about the advantages/disadvantages of different types.
A wheelie case means you don't have to carry it but will have a lot less internal space and lacks useful external pockets. I'd also be concerned that it won't fit under all seats; I don't regularly fly Easyjet, but on other airlines, the spacing of the seat frames is sometimes smaller than the space needed for a bag. They'll still allow it on because it's within the rules, but you'll have to find locker space for it and in my limited experience, Easyjet passengers seem to fill the lockers a lot more than Jet2 and Ryanair, so it could be anywhere on the plane, so a bit of a pain if you need something from it mid flight, or when you get off.
Holdalls might be awkward to carry as they're quite heavy when full. Which means I'd probably choose a backpack as easier to carry, lots of pockets for organisation and can double up as a day bag for the beach or buying shopping while away if needed.