Buy tankloads of sunscreen before coming here. The Red Sea sun is merciless. There could be somesunscreen in a Massawa shop maybe but in that case it will still cost three times as much as wherever you're from and have expired three years prior. So just bring lots of it along with a good pair of sunglasses and insect-repellents and mosquito-net bednet. The latter can be bought cheaply in Eritrea and its called "Lamse" LAM-SUH locally. There aren't much souvenirs to buy in Massawa which are legal to bring out or even ethical to trade in corals, mother of pearl etc. But if you're staying in Eritrea for a longer while, presumably up in Asmara, your hosts will greatly appreciate a great basket of sea-salt from Massawa.
