Planning a short visit this summer to check out the Lake Chapala/Ajijic area for retirement. I halve a valid US passport. What else do I need for this reconnaissance visit? Visa?
You will enter on a tourist "visa" called an FMT aka "tourist card". There is nothing you have to prepare ahead of time. If you are flying in, they will hand it out during the flight and you fill in your name and other basic information. If you are crossing on land, ... someone else can take it from here: where do you get the FMT if crossing the border?
The FMT is issued at the INM immigration station as you cross the border, or at some locations, a bit further on at the first checkpoint. Be sure to ask for 180 days, even if your stay is planned to be shorter. That way, if anything causes a delay in your return, you avoid hassles and expense.
If you are driving, you will also get a temporary importation permit for your car. That will require proof of ownership showing the VIN and a major credit card in exactly the same name as the owner/driver. When leaving Mexico, be absolutely sure to stop and get customs (Aduana) to remove the 'importada temporal' sticker and give you a receipt; which you will keep forever. Fail to do this and you'll be prohibited from bringing another vehicle into Mexico and you may be fined heavily.
They are very strict about this procedure but nobody will enforce it on the way out; it is up to you to know the law and procedures.