great stay!
errenteria is a small satellite town to san sebastian. koldo's flat is pretty close to the centre - near to the banks, shops, market , train stations and bus stop you'll need. but i recommend renting a bike from koldo to get around! san sebastian is only twenty minute cycle away on a protected cycle lane - and you can cycle to donibane too. just check rates to figure out whether it's economical - buses cost about 2.50 a trip.
koldo is very knowledgeable and full of tips on where to go and knows a lot about the history and what's currently going on!