In the Body argument of the Activity, you should be able to add your variables to the string you put in there: "<html> <head> <title>Example document</title> </head> <body> <table> <tr> <td>"+ String1 + " " + String2 +" </td> </tr> </table> </body></html>"
Make sure you check the “IsBodyHTML” property on the send mail activity you are using and as long as the HTML is formatted properly (including the variables being passed) it should be displayed in the body of the email like expected. If you have specific issues you may post the code (or a snippet) and I could offer more help.