1. A computer system for generating custom template-based documents, the computer system comprising: a template storage device; and one or more hardware processors programmed, via executable code instructions, to implement: a template generator configured to: access a placeholder template comprising one or more placeholders indicating locations for insertion of executable instructions; receive executable instructions to be included in the placeholder template; and store, in the template storage device, a template including one or more sets of the received executable instructions inserted into corresponding placeholders of the placeholder template; a user interface unit configured to: generate one more user interfaces configured to display one or more selectable graphics each corresponding to one or more respective data objects; and receive, via the one or more user interfaces, selection of one or more data objects to include in a generated custom document based at least in part on a selection of the one or more selectable graphics by a user; a template selection unit configured to receive a selection of the template; and a template processor configured to: parse the one or more sets of executable instructions included in the template; execute each set of executable instructions, wherein at least some sets of executable instructions include instructions to access properties of the selected one or more data objects stored in one or more data sources; generate an output for each set of executable instructions; and generate the custom document by replacing sets of executable instructions in the template with the output generated by execution of corresponding sets of executable instructions, at least some of the output including properties of the selected one or more data objects and/or summary data regarding the properties of the selected data objects.