A head-mounted display device, comprising: an at least partially see-through display; one or more input devices including one or more image sensors configured to collect imaging data of a physical environment; one or more communication devices; and a processor configured to: output for display on the display a mixed reality experience including one or more virtual objects superimposed upon the physical environment; transmit the imaging data to a server computing device via the one or more communication devices; receive, from the server computing device, an identification of a virtual content item display opportunity, wherein the identification of the virtual content item display opportunity includes a determination that the mixed reality experience and one or more physical features identified from the imaging data satisfy one or more spatial and/or temporal constraints; transmit a request for one or more virtual content items to the server computing device based on the identification of the virtual content item display opportunity; receive the one or more virtual content items from the server computing device; and output, based on the identification of the virtual content item display opportunity, the one or more virtual content items for display on the display at one or more respective dynamically positioned content locations in the physical environment that satisfy the one or more spatial and/or temporal constraints; wherein the identification of the virtual content item display opportunity includes a determination that the one or more identified physical features have a predetermined set of geometric characteristics, and wherein the one or more spatial and/or temporal constraints comprise one or more size constraints on respective sizes of the one or more virtual content items. an at least partially see-through display; one or more input devices including one or more image sensors configured to collect imaging data of a physical environment; one or more communication devices; and a processor configured to: output for display on the display a mixed reality experience including one or more virtual objects superimposed upon the physical environment; transmit the imaging data to a server computing device via the one or more communication devices; receive, from the server computing device, an identification of a virtual content item display opportunity, wherein the identification of the virtual content item display opportunity includes a determination that the mixed reality experience and one or more physical features identified from the imaging data satisfy one or more spatial and/or temporal constraints; transmit a request for one or more virtual content items to the server computing device based on the identification of the virtual content item display opportunity; receive the one or more virtual content items from the server computing device; and output, based on the identification of the virtual content item display opportunity, the one or more virtual content items for display on the display at one or more respective dynamically positioned content locations in the physical environment that satisfy the one or more spatial and/or temporal constraints; wherein the identification of the virtual content item display opportunity includes a determination that the one or more identified physical features have a predetermined set of geometric characteristics, and wherein the one or more spatial and/or temporal constraints comprise one or more size constraints on respective sizes of the one or more virtual content items. output for display on the display a mixed reality experience including one or more virtual objects superimposed upon the physical environment; transmit the imaging data to a server computing device via the one or more communication devices; receive, from the server computing device, an identification of a virtual content item display opportunity, wherein the identification of the virtual content item display opportunity includes a determination that the mixed reality experience and one or more physical features identified from the imaging data satisfy one or more spatial and/or temporal constraints; transmit a request for one or more virtual content items to the server computing device based on the identification of the virtual content item display opportunity; receive the one or more virtual content items from the server computing device; and output, based on the identification of the virtual content item display opportunity, the one or more virtual content items for display on the display at one or more respective dynamically positioned content locations in the physical environment that satisfy the one or more spatial and/or temporal constraints; wherein the identification of the virtual content item display opportunity includes a determination that the one or more identified physical features have a predetermined set of geometric characteristics, and wherein the one or more spatial and/or temporal constraints comprise one or more size constraints on respective sizes of the one or more virtual content items.