Located about 20 minutes from Hakone station by bus is one of Hakone's best day use onsen(hotsprings). Here, day users can bath all day for 1,200 yen while soaking in the surrounding nature. Although this onsen is not unisex there's a family bath just across the parking lot where everybody can enjoy bathing together. I highly recommend visiting Tenzan onsen on Weekdays as it is almost never crowded. Getting here is too easy. After arriving at Hakone station just wait for the free shuttle bus which comes every 20 or 30 minutes.
Recommendation: I recommend bringing your favorite book and a pair of pajamas or a robe. Tenzan is not just an onsen facility. It's also place full of ameneties with a wide range of restaurants and relaxation areas. The relaxation room overlooks a river where you can relax while reading or sleeping. They have an excellent shabu shabu restaurant which serves up some of the best tasting beer you can buy on tap. They also have massage chairs and a music room, a veranda and bar ! Tenzan is the kind of place you can either enjoy with family, friends, or even solo. I always like to go solo here because I can do whatever I want.
Another recommendation: Buy the soap. Tenzan makes its own hinoki soap which is made from its own hinoki trees. Also, in the souvenir shop, you have a chance to sample drinking the onsen water from a tap. For a few hundred yen you can purchase a bottle, fill it up and take it home with you. The water is alkaline based so it works wonders on the skin especially as a smoothing agent and as an astringent