Luggage and bags:
Small rucksack for your day trip in the cloud forest
Clothing/Shoes/Weather Gear: Bring light cotton cloths, a sweater or jacket for the nights, good walking shoes, and a raincoat. Don't forget your sunglasses and head protection.
Toiletries and Medical Supplies: Insect repellant with DEET, good protection for the intensive sun (factor 20).
Medical precaution: hepatitis A and B.
We didn't bring anything against malaria.
Photo Equipment: Bring lots of films or memory cards and a big zoom lens (min. 300mm or 10x optical zoom) for those wildlife shots.
Don't forget your video camera for all those rain forest sounds and active wildlife
Camping/Beach/Outdoor Gear: Bring a flashlight because at night it sometimes happens that there is no electricity for short periods
Miscellaneous: Bring an adaptor if equipment is not 110 Volt and american standard.
Make a copy of your passport and drivers licence and keep the originals in your hotel safe during day trips.
Luggage and bags:
Soft side bags
Clothing/Shoes/Weather Gear: Sandals that can get wet . The mud in the rain forest is pretty sticky. It took me a long time to clean it off my sandals, and it wasn't even that muddy when we were there.
No denim jeans - they are too heavy and take too long to dry.
Toiletries and Medical Supplies: Insect repellant. Leave makeup and hair dryers at home. Settle for a well-scrubbed look.
Photo Equipment: Be sure you know how to use your camera before you leave especially if you have just switched to digital.
Take plenty of film (I had 18 rolls for 13 days, and it wasn't too much). Take extra camera batteries. It is really irritating to have your battery die, and replacements will be expensive and may not be fresh. Take slower film for boat trips and places you will be outdoors in the sun (otherwise it may be overexposed), and take really fast film (at least 400) for the rain forest. It's dark in there. You may need a telephoto lens to take pictures of birds at a distance. You will need very fast film for that too.
Rain jacket is necessary because you are in a cloud forest! The humidity is always high and it can be wet, even in the dry season.
Camping/Beach/Outdoor Gear: The flashlight is essential, especially if you plan to walk away from Santa Elena, which is more like a downtown area. The road out of Santa Elena doesn't have street lights and can get very dark at night.
It is "cold" in Monteverde, especially on the many days when it is raining. By cold I mean about 10 C with no sun and very damp. This might not seem to cold when you come from weather that is -5, but after spending a couple days at the coast of Costa Rica where the sun is warm and the temperature is in the low thirties during the day and mid 20s at night, it seems colder.
Also being very wet it is better to have shoes or hiking boots than sandals.
Toiletries and Medical Supplies: It would be good to have sunblock and bug spray just incase as this is not the place to take a quick drive to the closest store! (see transportation tip) There are items available, but I would think that the prices are higher than elsewhere.
Photo Equipment: You will want it, but make sure to have some lens cloths to wipe all that moisture off!
Camping/Beach/Outdoor Gear: Rain gear is very advisable. It rains here more than half the days of the year.
Light colored tees
Light pants or shorts
Comfy Hiking Shoes
Toiletries and Medical Supplies: Bug Spray or Lotion. They manage to nip at you, even under long pants.
Photo Equipment: Tons of Film! Higher speed will be needed for some of the darker areas of the rain forest. Zoom lens will also be handy for the sloths that are way up in the trees.