Google Sites allows you to add a range of elements to your page. Instructions for adding images, videos and documents are given elsewhere on this Site. The principle for adding other items is the same though. Have a look in the Insert menu to see some of the other things that you can easily add to your page.
Presentations, spreadsheets and forms are all part of the Google Docs family and are added in exactly the same way as a document.
Picasa web slideshow is a tool for displaying a set of photos. Instead of adding, say, ten photos to your page and possibly making your page very long, they can instead be grouped as a slideshow using Picasa as in the example below.
Insert menu
You will need to create a separate account with Picasa, although you can use an existing personal Google account for this. Once you have created the slideshow in Picasa, choose Insert > Picasa Web Slideshow and paste in the link to your slideshow from Picasa. Choose a size for the pictures (the one above is Large-400px and any other settings you want, then Save.
The bottom of the Insert menu has a section called Gadgets. The first three entries here allow you to display lists of things from other pages in your Site. They relate to particular types of pages that you create when adding a new page to your Site. These gadgets allow you to display a summary list of the activity on those pages without cluttering the current page. They are particularly handy for allowing people to quickly see if there have been additions to those pages that they may want to go and view.
Text boxes are containers for text. They can be positioned on the page and other text - in the main body of the page - can be set to flow around the Text box. Think of sidebars and information boxes in text books where special information is displayed without distracting from the flow of the main piece of text.
More... will open a new window with a long list of other Google Gadgets that can be added to your page. Care must be taken in using these though. While some of them have been developed by Google, many have been written by third parties. They are categorised to make searching them easier and there are many that you may find useful.
I've inserted a Google Trends gadget below as an example. This can be configured for nearly any parameter you want. Try comparing the relative number of searches for avian flu and swine fever when they were each at their peak.
Other gadgets have been added on the right as further examples. If you find gadgets that are useful to you, make a note of them in the Comments section at the bottom of the page. Sharing the name and what you use it for can help colleagues find opportunities to extend and enhance their Sites too.
Bank of England Finance News Gadget
He Kupu o te Ra
Royal Society of Chemistry - Journals Search
Education Blog Search at Edublogs