There is no Admirals Club at LAS. The Centurion lounge is very nice, and available to any Amex cardholder for a $50 fee (or using the free card you get following a stay at several Las Vegas hotels when paying on Amex). Well worth it for a reasonably long stay.

There is now a train to T3 from the D Gates, but I'm not sure how it works going there and back, or getting access to the lounge, if you don't have a flight departing from T3.

D30 is the best AC in my opinion at MIA. Not massively inspiring, but quite airy, plus it has showers.

Also, at JFK, FL at both ends if you arrive and depart from the innermost concourse, or have the time (allow a good 10 mins round trip) to transition from the further away concourse to the inner one. If you're really pushed, or don't feel inclined, there's a (far inferior) AC on the furthest concourse, by the escalators.