Hmm, I'd say ちょっと出かけてくる(chotto dekakete kuru) or すぐ戻ってくる(sugu modotte kuru) is natural. And if you add ね(ne) at the end of each expression(出かけてくるね/ すぐ戻ってくるね), it will sound more friendly. You can say 行ってきます in this context by the way, but it sounds like it will take some more time than the above expressions.

ittekimasu, It doesn't matter what time you are leaving, If you wanna say "I'm gonna leaving a while, but will come back soon and see you in a few minutes", you always can say ittekimasu.