For example, email them personally on Saturday and explain that you’d like to welcome them with a small token of appreciation. Confirm or obtain their mailing address and send them something from your area, like a bar of local chocolate or coffee.
Either in that email or in a week or two, email to ask whether you can save them time with setup, or whether they and their team (or family or whatever) would like an interactive guided tour from you.
If your service has any complexity, offer to optimize their setup by hand.
Intentionally lose a small amount money to deliver the best service that this customer has ever experienced. Have fun with it.
See whether it translates to referrals or a tweet.
It may not be cost-effective with 100 or 1000 customers (or it may be…), but it’s amazing with 1 or 10.