My friend and I are flying to Costa Rica from May 12th 2016 to May 21 2016. We will be staying in Arenal from the 12th to the 15th. On the 16th we will be taking the taxi-boat-taxi to Monteverde. We plan on staying from the 16th and leave in the morning of the 18th.
Should we visit Manuel Antonio on the 18th till the 21st or is there a better place to see near Monteverde. It just seems like it takes a long time to go from one place to the other. Another option we were thinking of was going back to arenal and finish things that we may not have had the chance to do.
What do you guys think? Any ideas would be helpful.

If you like beaches, you could drive to one on the northwest side, such as Flamingo or Conchal (http://www.resortflamingobeach.com/Map-and-Directions), there's lots to do and see in the area (3 hours drive from the Arenal Lake area).
If you were thinking Manuel Antonio, I wouldn't bother, but would go a bit farther south of MA and visit the Uvita/Dominical/Marino Ballena area (you could even take the gorgeous drive back Hwy 2 to the San Jose airport).

MA is a long way from where you'll be unless you fly. I just returned from CR last month. I LOVED Arenal but was pretty bored by MA.

You could do MA, but it is a long way. Also might stay somewhere like Selva Verde or maybe the central valley - you could go to the La Paz Waterfall Gardens, Poas Volcano, the Doka Coffee estate tour, etc.

Is the town of Puntarenas worth visiting?

All great ideas. I loved the idea of Uvita/Marino Ballena area. Interesting beach and the idea of taking a scenic return are things that interests me also.
Poas Volcano and La Paz Waterfall is great because it'll be on our drive back to San Jose from Arenal.
I will look into the options and see what I choose to do. Thank you for the rapid response, I still have 2 more months to decide. I'm so excited for this trip.

But Uvita is farther than Manuel Antonio?

Thought the beach at MA was very nice. The beaches farther south disappear during high tide, something to consider.

Thanks all. Besides nite in San Jose before/after we have 2 nts in Tortuguerno area, 3 nites at Bosque De Paz lodge, 3 nites at Arenal Obs Lodge and 3 nites at Hotel Punta Leona. Have several river cruises lined up. Leaving in a week! Probably will add the Cano Negro cruise.

My fault. Posted on wrong thread.

No problem, r; I have done that myself.

Living in Florida, you would think i would consider about the high and low tide but i forgot about it. mlgb thanks for the tip. And rsnyder, it is no problem. I would love to hear your experience in Costa Rica, the good and the bad. The more i am prepared, the better. I have also watched several videos in costa rica and read tons of articles.

I never heard of the southern beachs disappearing; they are huge expanses of sand, it would take a lot to reduce them.

You can't walk out to the Whale Tail, Domincalito is also pretty much gone.

Maybe you could mention beaches between Dominical and Uvita that don't shrink to a minimal width at high tide?