Is there a way to notify ONLY the classmates who attended the reunion to complete the survey?

Yes. And no. There isn't a way in the survey itself, but... you can set up a survey (don't make it active so it won't show up in the menus) and then use Email The Class to manually select those who attended the reunion for an email containing a link to that inactive survey. You can find that link address by using Edit Site pages and Edit Link to look at the survey page's Open Page Info and Optimization to show the web address.
Make sure that survey page is restricted to logged in classmates only since they'll have to log in to take the survey.

We've done a couple surveys after the reunion and included all classmates.

Rationale: We want input from those who attended AND those who didn't.

ATTENDEES: what did they like, what do they want to do different next time.

NON-ATTENDERS: We ask a specific question--why didn't you attend (schedule conflict, not interested, OTHER)

ALL CLASSMATES: (1) What time of year works best (give several months to choose from --we offer June, July or August) When our group is older, there may be interest in homecoming weekend in October. (2) What activities interest you

I can see the advantage of surveying only those who attended, which is what I am TRYING to do, with no success.

AFTER I get that one to work, per John's instructions, I am going to make one for ALL classmates, asking some of the same questions I asked 5 years ago for the next reunion, e.g. --
-----what time of year?
-----set down dinner VS hor d'ouvers [sp] {never can remember how to spell that!}
-----one day event Saturday night or have mixer Friday night as well?

I feel it is important to 'segregate' comments about the reunion 'as held' from those who did not attend. Those who did not attend will have no knowledge of what actually took place, and five years ago, we had comments from many who did not attend. Their 'opinions' don't count for what occurred, as far as we are concerned, because they were not THERE.
Again, their feedback IS important, and sought, for the next gathering.

If you are using the Reunion Planner (I think the Event Planner does this too), you can go to Reunion Planner, REPORTS, then at the bottom of the page, there is a "Email the following members" button with a checkbox you can choose to activate saying "Answered Yes for attending, and have registered (63)". This will pre-load the EMAIL THE CLASS with those classmates who registered for the reunion. You can then add additional classmates who came without registering or remove classmates that didn't show up.

You can then combine that email with the hidden survey talked by John to get the desired results of emailing those that attended in order to get a survey response.

Is this what you are looking for?