I think 楽しみ is more likely a short version of 楽しみだ/です there. It would be an interjectional usage of the stem of a na-adjective, just like 残念(だ/です)!, 好き! or すてき!.
It sounds to me as お楽しみ(中)ですね!, i.e., it's talking about the addressee(you/them), not the speaker(I).