The Enshi-Lichuan area of Hubei is little-known by both foreign and domestic tourists, and one of my hidden gems of China. Though it's a bit tough going for non-Mandarin speakers without a guide or interpreter. As for a stopover to break up the 21 hours to Shanghai, how ballsy are you?

Easy Plan A: K698 Enshi to Nanjing, arriving approx 13:30. Rest of afternoon and overnight in Nanjing, also much of next day. Bullet to Shanghai late afternoon of Day 2. IMO, Nanjing is definitely worth 1 to 2 days though it's often passed by. Train trip still 15 hours though.

Ballsy Plan B: Stay the night in Enshi instead of taking a train out. First thing next morning, get bus/minibus to Yangtze Port Badong (will take about 3 hours). Take early afternoon hydrofoil down the Yangtze Badong - Yichang (takes about 2.5 hours with last 45 minutes completed by ticket-included coach since you have to disembark just above the 3 Gorges Dam. Rest of afternoon and overnight in Yichang, which is a rather pleasant small city. Next morning D3008 Yichang East- Shanghai arriving 15:44.

Meatier Version of Ballsy Plan B: Stay night in Enshi, bus/minibus to Badong as above. Afternoon join-in trip to Shennong Stream (tributary of Yangtze). Overnight in Badong. Second morning take first morning hydrofoil to Yichang, arriving w/plenty of time to catch D3074 at 13:40 arriving Shanghai 21:55.

The B Plans might be tricky to carry off if you don't have some reasonable Mandarin skills.

A final possibility Plan C is Wuhan, though it's such a big sprawling polluted mess of a place with subway construction underway everywhere, and can snarf up time just getting about. IMO, the best sight there is the Hubei Provincial Museum (free but closed Mondays). It would be possible to overnight train from Enshi to Wuhan (get a train going to Wuchang station, not Hankou!), arrive morning, taxi to Museum and Lake area, then taxi to new Wuhan station at take an afternoon D train to Nanjing or Shanghai.

Word of advice: you say "most nights" you will be traveling by sleeper train. I hope you don't plan on lots of consecutive nights on sleeper trains, without staying in normal hotel, guesthouse, or hostel lodging. This will get exhausting after only a couple of days or so as the quality of rest is never as good as a sleep in a fixed location. At least try to avoid consecutive nights on trains.