One day trip to unzen no stay at night?

Is it worth it to go to unzen without staying at the hotel?

Can I access the main attractions without paying a spa package+night?

send e-mail.
http://www.unzen.org/e_ver/index.html

You don't need to stay the night in Unzen if you don't want to. We drove to Unzen from Shimabara, walk around town for a couple of hours and continue our journey to Nagasaki.

Unzen is relatively small and can be toured on foot. The main attraction in town is Daikyokan Jigoku (Unzen Hell) which is just behind the Kyushu Hotel. It can be covered in less than 30 min. loop walk.
http://www.unzen.org/japanese/map/pdf/unzen-map.pdf

There are public bath facilities if you want an onsen experience.
http://www.unzen.org/japanese/higaeri/

Many hotels in Unzen also offer daytime use of their onsens for non-resident guests. Some have onsen + meal deal. Click on the hotels in the following list to take a look
http://unzen.org/e_ver/hotel.html

The main attraction in the town is the hot springs and one of the first golf courses in Japan. When in the area, I usually drive up to Nita-Pass and take the cable car to the top for views of the peninsula. From the top there are trails to do a bit of hiking, including back down to the bottom.