All good suggestions so far. I'll add a few specific ones I have loved or disliked...
- Doggie play pool for hot days. Here's the ultimate doggy pool - bone shaped (

) used for hot days + as a therapy & exercise pool for clients. Poca is the black dog at the back of the picture. Taken 4 years ago. She still won't swim or jump into a pool. She's a wader through and through.
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- Liberal drop-off and pick-up hours for boarding. I hate not being able to get my dog when I want her.
- A store so I can pick up items I need when I drop off or pick up. Things like favorite treats, bullies - stuff we use a lot and run out of. I'm past the stage where I care about the toys, leashes, etc. they stock any more. I have a closet full of that stuff I don't need.
- I no longer like cage-free boarding -- too much potential for issues, even with someone right there sleeping in the same room.
- A vet on call for emergencies + available on certain days to do vaccinations and routine check-ups.
- On-site groomer
- On-site trainer to work with dogs for free throughout the day + offering classes & personal training sessions for a fee. Having someone around who has extra knowledge for resolving issues is really worth a lot to me.
- canine massage class - I attended one of these. It was great.
- Photographer who comes in once a month to take portraits you can buy
- Periodic parent days so that I can see how my dog interacts with other dogs and meet the owners of the dogs mine really likes. Sounds foolish, I know, but it's fun to compare notes with the other owners.
As others have said, though, the most important thing is staff who really know dogs and how to manage them properly, which means keeping them safe and engaged. I've seen the daycares run like dog warehouses and they suck. I want low staff to dog ratios and eyes-on-dogs at all times.