just do your phpExcel stuff like you normally would. When creating the output object, specify "php://stdout" as filename.
This will cause the output to be sent to stdout. Ofcourse you want to capture it in a var. This can be done with output buffering:
$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel2007');
$data = ob_get_contents();
now your excel data is in $data; Please be aware that this will only work if output buffering is disabled :) Furthermore, errors from phpexcel also get captured by the output buffering.
I think PEAR has a mail module that should enable you to create an email with attachments.