Some thoughts in no particular order. 
Have a really good website. Clear, easy to navigate, usable in both desktop and mobile format.
If I was looking for doggy day care, I'd want to ensure you were licensed and that staff had animal care qualifications.
We've previously used dogwalkers and they also offered a keyholding service (very useful) and were CRB checked (I know the name has changed but you know what I mean).
Definitely offer weekends - would be a godsend for weddings, family dos, DC birhtday parties etc when it's useful for dog to go somewhere safe and quiet.
Fencing must be secure (really, really secure!) to cope with both burrowers and leapers! As part of the induction, ask clients to pre-register with DogLost so that if dog does manage to escape, immediate action can be taken. Clarify vet issues. Will you administer tablets? Ensure clear lines of responsibility if dogs taken ill/have accident during day. Can you offer drop off service to vets for things like routine appts for vax, worming, or similar?
Offer a variety of payment methods e.g. bank transfer, debit card, PayPal, etc. Make it as easy and painless as possible for clients to give you money, and minimise absolutely any cash handling (in fact, you should be able to be completely cashless).
Being blunt, I can't see the point of the agility field. Everyone I know who does agility insists on training their own dog so unless you are Mary Ray planning a career change, I wouldn't want someone else "training" my dog and having enough staff to do this 1-1 with dogs would suggest enormous fees. What staff/dog ratio are you planning?
Feeding - will you feed, and if so, will you cater for special diets, raw, etc. Would you provide or would clients?
Premises - if it's a farm, most farmers are so used to their own surroundings they don't realise that not everyone is surgically attached to their wellies. Drop off area should be well - lit, and not too muddy, as clients iwll be in their work clothes. Consider offering a drop off point at motorway services at nearest junction - could be convenient and reduce peak time traffic. Consider traffic management at peak drop off/pick up times! Parking, turning etc.
From a business perspective, understand your costs - you simply cannot do enough work on this. Don't underestimate the time involved with managing staff - training, customer care, record keeping etc.
Offering overnights to your regular clients seems like a sensible "add-on". Will you also have a small shop on site - you could sell things like toys, leads, collars, etc.