Around the island in a day

Absolutely, though unless you are tremendously pressed for time, I'd advise you to break the trip up to experience what the different parts of the island have to offer.

On our first trip, my wife and I spent the first night in SJ, and enjoyed Viejo.  In the morning, east to El Yunque, then south to Humacao, then off the highway and meandered along the 2-lanes until Arroyo (or thereabouts) on the south coast.  From there, west to Ponce, which we used as our "home base" for the rest of the stay, venturing to the west to La Parguera and Mayaguez.

We were struck by the tremendous geological diversity on the island.  San Juan beach gave way to Yunque rainforest, while Humacao reminded us of our own Shenandoah Valley.  Finally, the southwest (Parguera) is dry much like west Texas.  I regret not traveling further up the west coast and the hills and rivers to be found there, but we simply ran out of time.

So, yes, it is certainly possible.  The island is 100 miles side-to-side and 35 top-to-bottom, for a circumference of 270 miles.  Very doable.  But if you can, look for places to spend the night to break up the trip, so you can get a better sense of the place.

My thought was do  an around the island just to see what area we want to explore more. We will have 2 weeks using Fajardo as our base

Yes BUT, you need some local knowledge. First, the GPS does not tell you the best roads.  Second, depending on where and when, the traffic is WAY CRAZY!  Going through Patillas on the weekend is an event to remember.  There are many cars precariously parked near bars with live music and the dancing spills out on to the street.  Going toward San Juan at rush hour is, well, like any rush hour any where in the world.

It is very doable but you may have delays due to traffic. Typically I advice that people do an airbnb like arraignment for a day or two in each part of the island and the central mountain towns also. Get a flavor and get to see some of the beauty of each part of the island. Then return looking at the two or three places that take over your mind.

And if you go up the mountains you need to be extremely cautious. Roads are tight and winding, most are not bigger than a one lane. Locals will drive up and down those roads like speed demons.

Take your time and enjoy you don't have to see everything in life ne day.

In one day.....I don't know, why would you want to do that if you're sightseeing?????

good advise from all above - take your time, enjoy the island at a slower pace.   And watch out for the crazy Puerto Ricans that drive like maniacs.

Thanks
Doable not advisable. Still in planning stages.   Divide  sounds like a better plan. This I going to be a budget trip.

So far we planning on a trip out to Culebra for a day (when we can get  ferry tickets), the electric  boat bio bay tour, the observatory and maybe Yunque.

Our flight arrives at 4  by the time we get the rental car from Charile could we make it to the commissary n Ft Buchanan before they close at 7 or make the trip the next day?

My advice do not rent car from Charlie. Go with Enterprise or any other of the major ones. I had a bad experience with them, first car they gave me had bad tires and the second one was old and you could see it lacked all the maintenance. Plus if you have car trouble they do not offer road service.

I would never rent from them again. Check their ratings in the wwb.

Aldin thanks for the heads up

yes, get a car at the airport.  Take in the rain forest - a must do.  The southern coast is nice, but we settled on the north side.

Also, a drive accros the island thru the mountains is worth the trip.  Allow extra time as the road may be slow, daytime only.

Can you drive thru the rain forest?  Our friend that is comin  with us has beat lung cancer twice, but can do a whole lot of walking/ hiking. It gets tired easy.

You can either stay to the left on 191 (the road into Yunque) and it will carry you up to the mountain, or go to the right to the visitor's center (and pay a modest fee).  Might be easier on your friend to do the visitor center.  At the center you will learn about the trails up the mountain (on the left), some of which are not very taxing at all.

On the left, the road winds up quite a ways, through some sharp turns.  There are a few parking lots up there which fill quickly, so you will probably want to get to El Yunque early.

Rain forest is a set of hills and mountains, there are access roads thru some of it but to appreciate it fully you have to park and take the hicking paths.
Up hill drive with cliff on one side, pot holes and narrow road with some areas being too narrow for two cars.

Roads with 3 digit numbers typically are barely wide enough for trafic in both directions. 4 digit roads are narrower.

Stop at information desk and get a map of the rain forest.

After living here for a while, we use Dollar Rental and have had no issues with them.  And to answer your question about Ft. Buchanan, yes you can make it there before seven, but with an arrival time of 4:00 p.m., getting luggage and rental car, expect to leave airport by 5:15-5:30.  Ft. Buchanan is only about a 20 minute drive normally, but this will be end day and expect to be sitting in traffic for quite a while unless you are coming on a weekend.

Enjoy the Island, it is very beautiful and diverse.