J.,
If your folks don't want a party they probably to want or expect gifts either. Inviting them out to a nice dinner if a fine idea, but I wouldn't say the occasion is for their retirement.
I am about to semi-retire, which means that I may be leaving my full time job (which requires a full time person) to work at another company. I enjoy some of the people I work with and actually don't care for a few. So I have said I don't want a party or any fuss when I leave. I say this because a couple of my co-workers are very loud, constantly trying to talk over each other and talk about very private matters in public. My theory, I don't break bread with people that annoy me or socialize with people who I have absolutely nothing in common.
Blessings......