And all we ask in return is a trip report! ;-)

Will do, hopefulist!

I like the type of bag that Monica has posted. They are super lightweight. My test for bag lightness is to do a one-handed bicep curl with it! Those convertible backpacks usually weigh more.

Depends on the bag, mlgb - my Porter is the lightest bag of it's size I've come across and the suspension system is surprisingly good for such a light bag.

passports
money
underwear
pair of shoes
sun screen
DEET product
a few shirts and shorts, zip off legs work great
basic toiletries.
1 camera

Despite the weight, sunscreen from the US will be approximately 1/4 the cost it runs in Costa Rica...really.
And I find San Jose weather during the evening to be chilly, so long pants and fleece or jacket/sweater plus a long sleeved cotton shirt should keep you comfy. All those can also be your airplane clothing, saving valuable luggage space.
Carol

or buy the long sleeve locally.

We've done the 30 lb limit plane ride as a family a couple of times. We have rolling light weight duffle type suitcases. And the flight is usually towards the end of the trip. What we've done is redistribute the suitcase weight before the flights. The adult clothes and shoes will definitely weigh more, so the way you pack at home may not be the way you will pack before your Sansa flight. The night before we fly on Sansa I change up the suitcases based on dirty clothes, and weight and shoes etc so that everyone has about equal weight in their suitcases. We also always use the packing cubes (we got ours on ebags.com) and I now would never pack without them. They keep everything organized, both by person and type of clothing and then as the trip progresses, I put dirty clothes separate from clean clothes.

We do the zip off pants for my husband and son, mainly for activities like horseback riding or if it gets cooler at night. Myself and my daughter would wear capri pants.

And like others have said, wear your heaviest shoes on the plane and put items in your carryon bag so your suitcase is lighter. Things like books, cameras, flashlights, electronic equipment. Even shampoos and other items that may put it over the limit.

My kids wore keens or crocs the entire trip. If sneakers get wet, they tend to stay wet.

We also tried to have laundry done once during our trip, but that was on a 14 day trip. I think with 9 days you could probably do without. But most hotels will have that service available for you.

My kids were 7 and 9 on one of our trips, and they really enjoyed Costa Rica. Including the small plane rides! Have a blast!