、。
Today, shopping in town — doubling as an errand for grandma.
Literal
Today [topic-は], grandma [of-の] honorific-errand [object-を] combined-with town [at-で] shopping.
Diary-style noun-ending sentence — no verb, just the activity named as a fragment with an implicit 'did' or 'am doing.' ばーさん is an affectionate, slightly childish elongation of おばあさん 'grandmother,' dropping the honorific お- and stretching the vowel. お使い retains its honorific お- even in casual speech because the base 使い has become inseparable from the honorific — similar to お金, お茶. ~を兼ねて 'combining with / serving the double purpose of' links two activities as simultaneously fulfilled.