- You can develop web part but not Visual web part provided by SharePoint 2010. But that web part should communicate within a Site collection.
You can deploy below things in Sandboxed environment:
- Deploy a new list definition with list item event receivers.
- Deploy a list definition with list event receivers.
You can create event receiver classes within sandboxed solutions for events that fire on list items, lists, and individual sites.
- Also you can Create a content type.
- Create a new ribbon element.
- Create a new Site Actions menu item.
- Create an instance of a SharePoint list.
- Programmatically create a SharePoint subsite.