Ena isn't exactly a bustling area. Located along the Chuo Line, which takes the long way from Tokyo to Nagoya, it's most likely frequented by tourists moving between Nagoya and Matsumoto, which lies a bit north of the Chuo Line's route. There are plenty of smaller towns along the way as you wind through some mountains, and I'm sure there is a good amount to see along the way. My future trip plans include following this route to visit some old post towns - those interested in a few culture stops between Nagano and Nagoya can check out Kiso-Fukushima and Tsumago, fairly close together on the route through Gifu Prefecture.
Ena itself has a few sightseeing locations, namely Ena Gorge (Enakyo) and a modern portion of the Nakasendo trail (a restored version of the trail is accessible from Tsumago to Magome). Across the river from Enakyo is Enakyo Wonderland, an amusement park and garden.蛭川和田線) will get you there in about 15-20 minutes, and on weekends seems to run about once an hour. Listen closely for the announcements; you can ring the bell while you're crossing the large river and you can see the Ferris wheel on the left side. Note that you'll see the wheel for quite some time so wait until you cross the large bridge!
Park admission is 1100 yen (including the garden), pool admission is 1100 yen. Park admission plus a ride pass is 3200 yen, but it appears that this is for only 12 rides, not an all-day pass, Individual rides are 300 yen each.