First off what you're doing sounds like a pen. test/security test. It's very important when doing this kind of testing to ensure you have a signed off scope document from someone in authority at the company you're testing, as without that you could do something that the company wouldn't be happy with. Also without authorization from the company some actions will be in breach of local cybercrime laws.
So assuming that you've got a general authorization for "social engineering" style attacks, it would depend on your goal.
- Asking a user to open a file which gives you access to the internal network would be one option (using something like SET or Metasploit to generate the payload
- Ask a user to provide your their password. Depending on your scope again if successful you could then use this to access things like webmail systems to prove that attackers can gain unauthorised access to company information in this way.