Create Site Template using Sandbox Solution in SharePoint 2010

There are many way to create Site Template in SharePoint.I used below ways in my development experience.

1. Site Definition. - SharePoint 2010 (Site Definition project type in VS2010) - SharePoint 2007 (SharePoint Solution Generator create Site Definition & also WSPBuider, VSEWss1.3)
2. Site Provisioning (Programmatically) - You can create easily Site Template using this way because all controls in your hand (in your code). Add SPList, Delete SPList, Provision Web Part etc.
3. Site Template - For SharePoint 2010, Site Setting > Save as Site Template. Save as .WSP file in Solution Gallery. - For SharePoint 2007, Site Setting > Save as Site Template OR type /_layouts/Savetmpl.aspx in your web url. Save as .STP file in Solution Gallery.

Now I created Custom Site Template in SharePoint 2010 Sandbox.I refer this link.
You can create Web Template Feature in Sandbox Solution & give scope Site for this feature.
Given link will help for me so I hope it will help you.

Happy Coding!!!