Costs Ricans, who call themselves "ticos", are some of the friendliest people I've found in my travels. They like Americans. Costa Rica is also the safest country in Central America (and maybe Latin America). It offers fascinating scenery (volcanoes, water falls, beaches, giant leaf plants), jungle wildlife (four types of monkeys, tropical birds, butterflies), and superb touring. Don't forget the binoculars (several manufacturers make a 8x24 that weighs 10 oz.). The most famous birds are the quetzal (a god of the Mayas), the macaw (giant red/blue/yellow bird from the parrot family) and the tucan (the "Fruit Loop" bird). Listen to the jungle. You can hear birds make the "come here" whistle and the "she's cute" whistle. Some monkeys species howl and some squeak.
You would need a mountain bike for about half the places to see. I especially recommend getting front shock absorbers for your bike. In order to make roads passable in wet weather, they spread rocks on the road. Without shocks, I had to keep up a minimum speed (20 kph loaded), and lower the air pressure to 35 lbs. Even then, my hands started aching after a half hour, necessitating a rest stop. Front shocks would have solved this problem. (While rear shocks are also made, to date none of these bikes can carry a rack and touring gear.)
Humidity and heat were only significant on the west coast, but I found the climate tolerable. Also, I sometimes rode partly at night on the west coast since it was cooler. There's plenty of places to stop for cold drinks (and to get yourself wet), even on single lane dirt roads. All the villages, haciendas and large fincas (farms) have stores. Often, it's a family selling things in their living room. You can even ask if they'll cook a meal for you, if your hungry. These "facilities" make Costa Rica superb for unsupported touring.
Upon arrival in San José, get a map at the tourist office and buy a contour map for the country at a large stationary store (near corner of Calle 1 and Avenida 1.) The contour map is important because the regular road map does not show all the roads. For example, I road from San Jose to Quepos on the Pacific coast through a jungle dirt road (two day trip with 50 lbs of gear, 120 Km and 2 Km elev. gain, 3 Km drop). It was fascinating, yet the road wasn't on the road map. You also have the option of putting your bike on most buses and all trains, should you run out of time or wish to avoid a main highway. Trains are narrow gauge (some lines are electric) and fun. You may want to ride down from San Jose to either coast and take the trains back.
Food was the only disappointing part of the trip. Rice and beans for every meal (including breakfast!) with some meat (usually fried unless you knew where to go), plus often fried bananas (these were good). In contrast to, say, Mexico, the food is very bland, no variety and tasteless. No spices. There are many Chinese restaurants, but alas!, the food has been modified for local taste buds. So tasteless I couldn't eat it. There is no Sechuan, Hunan, Kung Pao or Mandarin anything in the entire country. In contrast, the tropical fruit drinks were just wonderful. I never heard of most of these fruits. Ripe mangos (called "mangas") were exquisite. Water is usually safe, but ask first (I did bring a katadyne filter for village water). Also, I tried to boycott beef because of their sorry practice of cutting down virgin jungle to grow grass for Brahman cattle.
Fenders are essential most of the year, and recommended even during the dry season. They're not necessarily to keep you dry, but to keep you clean. A warm tropical downpour will soak you with or without fenders, and it's fun to ride in. However, laundry facilities are minimal. Hand washing the jersey gets most of the sweat out, but not the muck spray. There are only two laundromats in the entire country, although you can ask around a village for someone who washes clothes. I used Zefal ATB plastic fenders ($18 from Bike Nashbar), keeping plenty of space between the fender and the tire since the fenders will vibrate. In addition to a typical toolkit, bring a small plastic bottle of oil (e.g. TriFlow), a freewheel removal tool that matches your freewheel (foreign bike shops probably wouldn't have it), spare rear deraileur cable, spare chain links, a few nuts and bolts (matching existing ones that may fall out), and a pedal wrench.
Note on front panniers: these significantly reduce handling of the bicycle when riding on dirt (unless it's hardpacked -not the case for Central America or most deserts). Even when minimizing the weight in the front (by placing clothing), the panniers greatly increase the inertia [=mass times distance to the steering axis] of the wheel, fork, and pannier assembly. This effect becomes even more pronounced on sandy roads, where fast feedback is required. Your arms moving the handlebar provide corrective feedback to keep the front wheel from going unstable. With panniers on, you simply cannot move the handlebar as fast as without panniers because of the increased inertia. In engineering terminology, this lowers the frequency response of the feedback, and the system becomes unstable (when the front wheel turns all the way and you're forced to stop). A friend touring in Canada states most dirt roads in Canada are hardpacked, and front panniers do even out the weight, a possible advantage. However, for the roads that are not hardpacked or paved, he ties his front panniers onto the rear rack.
Note on airlines: While some airlines charge extra for bicycles, others do not. Ask about bicycle policies before you buy the ticket. On many airlines, you are allowed two pieces of luggage, and the bicycle counts as one. The other was my Madden three piece rear panniers zipped together and with sleeping bag attached. You just lower the seat, turn the pedals in, and turn the handlebar. No boxing, no fuss. You should always have the handlebar (and the cyclocomputer) slightly loose anyway, to prevent damage during crashes or transport, and to make it easy to turn (without fidgeting for the allen wrench) for putting on trains or on top or bottom of buses when avoiding developed areas.
Don't rent or buy a bicycle there. The rentals are total junk, and you have to pay a 100% sales tax if you buy one there vs. buying one in the USA and putting it on the plane.
Prices are about $5 /day for a hotel (except San Jose), with your own bathroom/shower and hot water! Meals are about $2. It's actually cheaper to visit Costa Rica (including hotels, restaurants, and air fare) then to stay at home in the USA. Just put all your stuff in storage before you go. [OPTIONAL: I spent two months bicycling, yet needed at least 3 months to see all the things I wanted to. I spent 1 week backpacking, 1 week disabled after crashing due to unexpected sand riding at night, 1 week recovering from the flu, and 1 week in the mountains. I accumulated 2000 KM and 20 KM elev. gain.]
I and others have found Costa Ricans to be incredibly honest. Unlike Mexico and many other countries, you don't need to ask the price first. They will charge you the same no matter who you are or without ever seeing the price list.