Think about souvenirs for when you return from the trip.

Literal

Trip [from-から] return time [possessive-の] souvenirs [possessive-の] matter [object-を] think [preparatory-ておきなさい].

~ておきなさい combines ~ておく (do in advance) with なさい (polite command) — 'think about this ahead of time.' 土産 (みやげ, souvenir) is an essential cultural concept — bringing back souvenirs from trips is a strong social expectation in Japan.