What simple events can a graduate student club organise to network with former students, companies, etc.?
Simple ideas:


Seminar where a professor gives a talk on his/her current research. Provide beer.
Invite an alum from one of the companies you're talking about to give a talk on something new from the company. Provide beer.
Simple cook-out, which includes beer (even better if this is a recurring event, monthly, once a semester, etc.).


More involved ideas:


Mini-poster conference, where a quorum of students have posters about their current research. Alum can act as judges, if you want to award prizes. Provide beer.
Invite a famous alumus to give a talk. This always involves more work than you're expecting, but can have big pay-offs. Might necessitate actual cocktails (and possibly pay for the speaker, but if you play it right you should find an alumnus willing to do it for free).
Field-trip to a local engineering firm that may have something interesting to look at (or a tour). Drinks at a local bar afterward.