The Gemara (Shabbat 150a) derives from the words (Yeshayahu 58:13) “Vedaber Davar” that speech related to weekday matters is forbidden on Shabbat, but not mere thoughts. Indeed, the Shulhan Aruch (O.H. 306:8) rules that, strictly speaking, one may think about one’s business affairs on Shabbat. Nevertheless, he cites Rabbenu Yona (Igeret Teshuva), who says that it is a Mitzvah to specifically refrain from thinking of one’s affairs and to consider all one’s business as being settled and complete. Rabbi Rahamim Palagi (Yafe Lalev, vol. II, § 4) comments that Rabbenu Yona’s opinion was only with regards to ordinary business, but that he would have most likely permitted thinking about something that gives someone excitement and joy, such as planning a wedding or a Hilula.
The HIDA (Birke Yosef ibid.) writes that the Shulhan Aruch’s intention is that only thoughts are permitted, but reading a business document or the like which stimulates thoughts of business is not permitted. Furthermore, the Mishna Berura (306:1) says that thinking about visiting one’s investment properties is problematic and Rabbi Ovadia Yosef (Hazon Ovadia, vol. V, pg.152) writes that (thinking about) going to see one’s properties on Shabbat is forbidden.
Summary: One may think about one’s business on Shabbat, but it is a Mitzvah to refrain from doing so.