in march? can anyone tell us approx day time temps? humid?

Arenal is central valley plus it is the dry season so humidity is not a problem there. You might even need a light jacket or sweater in the evening. I would say to plan on 60's-upper 70's and possibly lower 80's. Doesn't generally get real warm there. Most likely the temp will be very comfortable day and evening. It is hard to be too exact. I was there this past Feb and I was very comfortable in a bathing suit in the evening while hanging out at the hot springs.. A week later a cold front came through and it was quite cool. March is probably one of the warmest months although, because of Costa Ricas relation to the equator, temps don't change much.

thank you, suzie!

60's during the day is cooler to me. Maybe I should bring a sweatshirt or two. I am already bringing a lightweight windbreaker type jacket with no lining and a slightly warmer lined jacket. Unfortunately, I don't know the meaning of 'pack light'.

Leave the sweatshirts home. You should be warm enough with what you already have planned. Chances are you won't even need those but it is hard to say 100%. If it does get down into the upper 60's it would be at night when most likely you will be snug in your bed.

I wish I could remember the month I was there. It was very hot during the day....must have been at least 85% if not higher during the day. It did cool down enough at night for me to use a LIGHT sweater.

hot fronts (?) can happen as well as the cooler fronts.
upper 60's to upper 70's is the average nighttime. Lower to mid 80's is average daytime. I guess I was unlucky the first couple of times because it was cool there. One of my trips (March) it was cool (had to wear a jacket all the time) and rainy. Must have been one of those cool fronts. Last time was just perfect (Feb) and around 80. I believe the other time it was a little bit of everything.
My comparisons are with the Guanacaste coastal area and the Osa which would make Fortuna seem cooler.