I love this way of using community lots, too :)
I must admit I have not worked a lot to combine functions/builds, but I often add a small café or some offices to existing lots in order to give some of my sims an office away from home etc.
The Kasbah Art Center in San Myshuno works as the original Arts center in combination with my Casbah Art School. + in the basement I put the region's Court House with separate entrance from the side of the building.
Behind the Orchid Go Go Lounge there is a backyard, big enough for a couple small businesses/offices. My sims will not benefit from features like retail, but I make sure they make some money.
In Magnolia Promenade I built a culture house called "Fun Factory" that includes misc studios that artists can use for free, and there is a bar and a stage with seats for a audience of about 25. The build also include a café, exhibition section and a spot for meet&greets that can be altered a bit as needed.
Back side of the library in Willow Creek's commercial area, is a small café. It's not operated, but whenever I want to bring a small group of guests there I can simply bring a cake and there is a coffee maker there.
If I want more guests etc, I'll just use the club feature.
In Oasis Springs I combined a Spa with a Lawyer office. The Lawyer will typically not have many clients at the same time, so she will just invite over 1 or 2 sims.
In the future I'll focus more on community services. My region hires Janitors and I might build them a small central at the corner of a community park or in some backyard. I also want a bigger business/offices build where the units can change over time. Perhaps a bakery on ground level, and offices on the next couple floors.
There's probably a few more... :)