Simply Edit your Google Site page
> Insert > More Gadgets
Select from a wide range of gadgets from the Gadget Gallery or Add the specific Gadget by URL.
> Insert > More Gadgets > Add Gadget by URL
You can also create your own Google Gadget and publish it to the Google Gadgets Gallery.
Some Popular Google Gadgets get over 20,000+ page-views per day which can result in viable business opportunities.
Google sometimes provide grants for further development of Quality Gadgets and Viable Gadget orientated Businesses.
Google Gadgets Editor
Google Apps Script is also a very powerful tool, where you can embed a GUI (Graphical User interfaces) with control over Google Sites, Google Docs, Google Calendar, Google Maps, Gmail and many more applications.
Calendar Service : Date and Time Picker
Library of Apps Script Snippets & Objects - LASSO
LASSO Google Calendar to Sheets
LASSO Google Contacts to Sheets
LASSO Google Drive Filetypes to Sheets
LASSO Google Drive Folders to Sheets
LASSO Google Sheets to Calendar
LASSO Google Sheets to Sites
LASSO Google Sites to Sheets
Sites Service : Email Template using a Webpage
Html Email Template