The Shulchan Aruch (113:8) addresses the case of one who recites the Amida in a public area and, as he reaches a point in the prayer where he is required to bow, a gentile walks in front of him with an idolatrous article, such as a cross. In such a case, the Shulchan Aruch rules, one should not bow at that point. Even though one of course bows to show submission to God, and not to the object of idolatry, it is nevertheless forbidden to bow towards that object.
In the next Halacha (113:9), the Shulchan Aruch discusses the subject of descriptive praises of God during prayer. In principle, we should not use any descriptions in praising God, whose greatness extends well beyond anything that words can convey. In the first Beracha of the Amida, we describe God as "Ha'Kel Ha'gadol Ha'gibor Ve'ha'nora" ("The great, mighty, awesome God"), and we are entitled to do so only because of the authority of the Anshei Kenesset Ha'gedola (Men of the Great Assembly), who formulated this text. Therefore, the Shulchan Aruch rules that one may not add any other adjectives in praising God in the Amida, for even the adjectives that we do mention are allowed only due to the authority of the Anshei Kenesset Ha'gedola.
The Shulchan Aruch adds that strictly speaking, one who recites other prayers to God, besides the Amida, may mention other descriptive praises, beyond the ones incorporated in the Amida. Nevertheless, he writes, it is preferable to use Pesukim from Tanach in praising God. The Book of Tehillim was composed by King David with Ru'ach Ha'kodesh (prophetic inspiration), and it contains many chapters of praises of God, and one who wishes to offer extra prayers should preferably praise God specifically by reciting verses from Tehillim.
Summary: One should not bow during the Amida at the moment when a gentile passes by carrying or wearing an object of idolatry, such as a cross. One may not add descriptions of God in the Amida that do not appear in the fixed text. One who recites other prayers may add more descriptions, though it is preferred to use specifically verses from the Book of Tehillim when speaking in praise of the Almighty.
