My ideal set-up would be something where I could volunteer overnight shifts one or two times a week.
I volunteer and work full time right now...have done so for years. I've found that my local hospitals offer lots of shifts that fit well around work and life and offer lots of patient contact.
First, weekends are always a solid option for the person who has a normal M-F 9-5 working gig. And, if you really don't want to give up weekends, or can't for some other reason, I've found I can sneak it in during M-F as well:
While most of the shifts are formally set for once a week during the day 8AM-Noon, Noon-4, 4-8PM, I've had pretty much everyone say it's okay (we're volunteers!) to slide it to the 5-9PM side if it fits a bit better. But perhaps that's not the norm - it could be because I'm older and don't look like I'd take advantage of it. And after you settle in and prove yourself reliable, I'm sure they'd let you pick up an extra shift so you have 8 hours a week. I met a new nurse on one of my assignments - she would do three shifts in a row to build up her hours quickly and to show potential employers that even though she was new, she had the chops to handle the long shifts.
The ED at my locations offer nighttime shifts as well - I prefer the 8-midnight over 4-8AM shift...and I'll be honest that I've never done the 12-4AM shift. No point in causing unneeded misery. Late night ED is what I've been doing lately to minimize the impact on my family.
All of the hospitals I've volunteered at have a formal application process for volunteering, a "training" session, shot records (TB test, vaccines, etc.) and then assignment based on openings they have and my area of interest.
Good luck!