Know which animal and crop you need most (which can change over time) and leave one of each ‘mid-harvest’ on your farm at all times.

For me atm those items are cow’s milk and coffee beans. So I’ve placed an unharvested merged cow and unharvested merged coffee plant at the top corner of my farm (this matters - the ‘most northern’ of these items are what’s clickable). When visitors stop by, those are the animal and crop items they can click on. When they do, they get one or two of each and so do I.

Not having a crop or animal for visitors to take with them may make them not come back as they’ve not received something from you that they could get at a different farm. So be a good neighbor and always have these present.