Some other sites in addition to the excellent ones mentioned by JJP:
Dry habitats: Flowering Porterweed around Cerro Lodge is good for Cinnamon, Steely-vented, Ruby-throated, and Rufou-tailed Hummingbirds. Green-breasted Mango, Scaly-breasted, and Plain-capped Starthroat also possible near the lodge. Carara is also nearby. The River Trail can be good for hermits, Scaly-breasted, Purple-crowned Fairy, and Long-billed Starthroat (all in natural settings).
Highlands and foothills:
Hummingbird and butterfly garden on the main road between San Ramon and Los Lagos- Watch for the sign on the west side of the road. Excellent for White-bellied Mountain-Gem in addition to several other mid-elevation species.
Near Poas- Hidden garden across street from the Freddy Fresas restaurant. Has feeders and several of cloud forest species.
El Toucanet Lodge, Santa Maria de Dota- Among the best hummingbird feeders in country. Species similar to Monteverde with addition of Scintillant.
La Georgina, Cerro de la Muerte- Roadside restaurant with great feeders for Volcano, Fiery-throated, and Magnificent.
Noche Buena restaurant, Irazu- Nice feeders for same species as La Georgina.
Arenal Observatory Lodge- Porterweed great for Black-crested Coquette, Violet-headed, Bronze-tailed Plumeleteer, and others.
El Tapir, main highway between Guapiles and San Jose, near northern boundary of Braulio Carrilo National Park- defunct butterfly garden with Porterweed that attracts Snowcap, Green Thorntail, Plumeleteer, and others.
Talari Mountain Lodge, along road up to Chirripo- Flowering plants on grounds good for Charming Hummingbird, Long-billed Starthroat, and Snowy-bellied. White-crested Coquette and Garden Emerald may show up.
Hope that helps!