What is the best marketing strategy (aside from digital marketing) for a physiotherapist who just started a business?

I gathered some of the information from the Internet in this answer for you. References in the end.

First you need to get exact answers to following question which require some thorough research:
1. Why does your audience need your services?
2. What propels your audience to seek your services?
3. Where does your audience go to learn about the services you provide?
4. Who (or what) influences your audience’s buying decisions?
5. What competitors are there nearby your business?
6. What they are offering?
7. What different thing you can offer than them that they can't replicate easily?
8. What is the current state of the industry you have business in?

Then you can follow some of these strategies including the strategies you mentioned:

Go for a referral system. Give discounts to those customers who sends more customers to you.

Ensure your contact information (on your website, business cards, and social media accounts) is current.
Use Google Docs to create an easy-to-complete patient referral form for physicians and other providers to complete on your website. Then, include the URL on your business cards and marketing materials.
Create a free Google Voice account to add a dedicated physician referral phone line. You can even use Voice to record a custom greeting and transcribe your voicemail into text, so your front office staff can pull referral information from an email and put it right into your physical therapy documentation software.
Take one day every several months to visit a handful of the largest physician groups near your clinic. Stop in and introduce yourself—and bring along a friendly smile and perhaps some goodies. You never know; they may have a patient who needs therapy that day, and you want to be top of mind.

Partner with local athletic trainer, yoga instructor, or massage therapist. Take them out to lunch and build a strong relationship that will lead to long term partnership. They will refer targeted customers to you.

Always follow-up your customers. Call or mail them asking about their health or reminding them about their monthly checkup. I will prefer to mail if not an emergency condition.

You can also go for webinars and workshops for medical students. This will position you as an expert in medical community. Self branding will help you get more customers than promoting your business.

Ref: https://www.webpt.com/how-market-your-physical-therapy-clinic

I'd love to find out a bit more about your business - such as what population you'll be serving, where your office is located (i.e. in a building, a strip mall, inside of another practitioner's office), and what your specialty is.

I've owned and operated over a dozen physiotherapy offices and can tell you that there's no boilerplate answer to how to market.

That said - I'd suggest you get very clear about who you plan to serve and be as specific as you can. Once you know who your market is find out where they are and how they are looking for help.

If you message me with more specifics about your business and business model I'll be happy to get more specific with an answer.

Best of luck!

Why are you so quick to discount digital marketing?

To grow off-line, you must go where your ideal consumers are. Think about where they hang out. Bonus points for finding places they naturally gather in large groups.