Following the "Hill"-river from Eupen to Signal de Botrange, the highest point in Belgium, you have a climb of around 500 m, the most you can get in Belgium. But: don't take this too seriously, with an average of 2% ascent, the "climb" is not that steep...
Nice walk through the wilderness of Belgiums "High Fens". Certainly the middlepart of the trail leaves you in de-humanised terrain, having the Hill river as your companion of the day
Don't do this after days of heavy rain: the fens would be re-shaped an an inpenetrable swamp
Eupen, busstation "Croix Noire" (333 m)
32U 290550 5610864
Botrange, Signal de Botrange
After the descent to the Hill river, you go right and take the road. After a while, this road turns into a dirt road. You cross the river, walking now on its right bank. Further on you walk on paths an dirt roads, following the river. Pay attention at the "Grand Bongard" (around km 13,5), where you cross two tributaries of the Hill river, ending on the left bank of the river.
From there onwards, you walk through the fens, meaning some swampy underground. You'll find some "bat paths" to avoid wet feet, but they are not everywhere you would expect them...
At 5km from the finishline, prepare for a bit of a disappointment: you have to march on a boring dirt road to reach your final goal, the Signal de Botrange, Belgium's peak. But: you can try to keep on following the river through the fens, but this is not the "official" path.
Public transport friendly
You can reach Eupen by train. There are certainly hourly connections to all major cities in Belgium. You can start walking from the trainstation, or take bus 394 to the actual starting point "Croix Noire" (7 minute trip). The bus is serviced by the TEC, the Walloon bus company. Attention: in weekends and holidays, there is limited service
You can return from Signal de Botrange to "Croix Noire" or "Eupen Bahnhof" taking the same bus 394. This takes you about half an hour.
Or you can park your car at the Signal de Botrange (plenty of parkingspace just next to the busstation) and take the bus to your starting point.