Reformatting layout of information in Browse Items

I want to add additional metadata to the thumbnail displays of items when browsing by items. Currently the thumbnail is shown with the title and description. Is there a way to display other metadata fields, and possibly the collection that each item is associated with?

Alternatively, can I disable browsing by items so that items are only browse-able by collection. Many of our items are similar, so we are trying to avoid redundancies (such as having to include the collection name - a location - in the title).

You can more or less completely customize what is shown on the items browse page by editing or creating your own theme.

You can get some information about customizing themes from the Omeka documentation. The file you're going to be working with is items/browse.php.