Hakone Travel Passes
Odakyū's Hakone Freepass, available at Odakyū stations and Odakyū Travel branches, is an excellent deal, covering the return fare to Hakone and unlimited use of most modes of transport within the region, plus other discounts at museums and facilities in the area. It's available as a two-day pass (adult/child from Shinjuku ¥5140/1500; from Odawara, if you're not planning on returning to Shinjuku, ¥4000/1000) or a three-day pass (adult/child from Shinjuku ¥5640/1750, from Odawara ¥4500/1250). Freepass-holders need to pay an additional limited-express surcharge (¥890 each way) to ride the Romance Car.
If you plan to combine Hakone with Mt Fuji, also consider the Fuji Hakone Pass (adult/child ¥8000/4000), a three-day pass offering discount round-trip travel from Shinjuku as well as unlimited use of most transport in the two areas.