Phu Kradeung was the 2nd national park in Thailand, its status was granted in 1962. The first was Khao Yai, for perspective. It's relatively unknown to foreign visitors but is popular amongst young Thais. It's basically a pine forest plateau and then jungle/thin forest on the slopes.
I think the reason not many foreign tourists get here is the location, it's nowhere near a beach or Chiang Mai and is a 6-hour slog from Bangkok with no nearby airport. It's one of the few national parks accessible by public transport, though. Buses between Loei and Khon Kaen take you to a small town nearby and from there you can get a songtaew to the park entrance for about 30 baht. The songtaews only leave when full (10 people?) so if you arrive at a quiet time you might have to pay more if you don't want to wait. Some (all?) Bangkok to Loei buses take you past the same town. Make sure you tell the bus conductor you are going to Phu Kradeung and they will drop you off at the right place for the songtaew.
There is a trail run here in October every year to celebrate the new season (park is only open October-May). You can run 9km (trailhead to campground) or 25km (trailhead to Lom Sak Cliff to campground). If you are a foreigner it works out as a very good deal because the race cost is only about 450 baht and for that you get a shirt, medal, 100 baht food coupon and free entry to the national park worth 400 baht (just show your race entry confirmation at the checkpoint).
web.facebook.com/PhuKradueng.RUN?_rdc=1&_rdr (edit 2022, this event has been acquisitioned by another company and they have expanded the format to include longer distances up to 100km, the price has also gone up considerably).
Phu Kradeung is not really a wildlife destination like Khao Yai, but there is a lot of elephant activity in places. The elephants only venture into the waterfall areas at dusk when the trails are officially closed. There are sambar deer at the campsite which are very tame, you can get close enough to scratch their neck. I've read that there are also occasional jackal, civet and boar sightings around the campsite area. There are pine trees all over and in the winter months the maple leaves fall onto the streams; both big draws for the Thais.
Let's start with the walk up to the plateau. You can put your GPS away - the path is about 10 metres wide with no side trails. There are snack vendors on the trail at 3-5 points on the way up and some sections near the top have wooden stairs. It is 5km with 1km of elevation gain. At the plateau you then have another 3km flat walk to visitor centre and campground. Total time should be around 3-6 hours depending on fitness. You have to start before 3pm (edit 2022 now seems to be 2pm, not sure how strict this is) and any slow trekkers should aim to start before midday if they want to arrive at campsite whilst it is still light. You can hire porters to carry your stuff, 30 baht per kilo.
Water is expensive as somebody has to carry it up there. 60 baht for a large water. You can fill bottles up at streams if you're going trekking on the plateau and you trust the water source. They rent good quality bicycles at the visitor centre for about 300 baht a day and if you want to go to the furthest cliffs e.g. Lom Sak it's a very long walk so bicycle is preferable. The campsite is huge! I think it is the biggest one in Thailand. Hundreds of tents. Only on holidays and long weekends do they need that many tents but I don't think they take them down in between; understandable as it would be a mammoth job (edit 2022: not many tents set up after covid closures). There are quite a few bungalows as well. I expect on weekends and holidays the bicycles and bungalows might be fully booked.
Once at the top you can more or less divide the trails up into two sections, the waterfalls trail and the cliffs trail. Most people only have chance to do one route because to do both would be a long way to walk in one day and most people go up Saturday and down Sunday. It is not possible to take a bike on the waterfalls trail as it is too technical. The cliff trail is the one 90% of people go for. There are 4 or 5 named cliffs (see picture below) but really the entire walk is along the edge of a continuous cliff so you get unobstructed views almost 100% of the time. Lom Sak Cliff is the famous cliff with the rock jutting out and the pine tree above it but it's also the furthest away - a 19km round trip from VC. On my most recent trip most people were getting about by bicycle, I only saw one group walking to Lom Sak Cliff compared to 4 or 5 groups on bicycles.
The waterfall trail is interesting; due to the topography you don't expect much but the waterfalls are very pretty. On my elevation map below you can see there's just enough variation in elevation to form streams and waterfalls. But a lot of areas on the plateau are boggy - there are quite a few ponds.
A Google Earth view of where I walked, with some labels overlaid: