The terrain is hilly.. It is hard to avoid climbs and descents unless you walk parallel to the river, as you note. Are you experienced hikers, or more casual, as I am?
I'm not sure about trails along the river. The map below does not show any:
map.wanderland.ch/?lang=de&route=all&bgLa yer=pk&resolution=2.5& layers=Wanderland&E=28 07740&N=1183645
When we stayed in Ftan, we always walked from Ftan to Ardez,. then down to that station and returned by train or bus. The old inter-village road is above the valley - Lavin- ruins of Gonda- Guarda- Bos Cha -Ardez- Ftan.
A nice scenic short walk is to take the cableway from Scuol station up to Motta Naluns, walk to Ftan Prui, take the chairlift down to Ftan Pitschen, walk to Ftan, bus back to Scuol station.
Sur En is a pretty litle town opposite Guarda, across the valley. Walk in from Giarsun. There is absolutely zero parking at Giarsun, if you happen to have a car. If you come in from the river crossing near Ardez, the climbs are more noticeable.
That map above will help for all trails. Since it is a topo, you can see that altitudes.
If you are serious about maps, this one is serious right back at you:
map.geo.admin.ch/?topic=ech&lang=en&bgLa yer=ch.swisstopo.pixel karte-farbe&layers=ch.bav.ha ltestellen-oev,ch.swisstopo.swiss tlm3d-wanderwege&layers_visi bility=false,true&X=17 2594.52&Y=812042.09&zo om=5
it shows the trails, if you turn them on from the menu, and blue are technical. Red are "mountain" - need boots and staff.. Yellow are smooth but may still have climbs and descents.
<<"Does anyone have a trail suggestions that stays mainly in the forest and
around the lakes for the national park. Not interested in technical
climbing or steep slopes.">>
I have set the map below to show the trail from Il Fuorn to Ova Spin, which is far from level, but is relatively easy as trails in the National Park go. When we took it, we noted that it was "suitable for handicapped."
Not sure which lakes you mean?
When were climbing it, we could not believe that. You start at 1800 meters- well above a mile high, and the air is "thin." We asked a friend later what "suitable..." meant. He said that meant it was wide enough for a wheel chair. It was.
All the park is at high altitude, which may affect your stamina. I pant a lot on the uphills.
The only other ones that I have been on that a are not too steep ( a relative term, mind you ) go in from S-Charl.
Even there, by the way, the parking lot is metered. ;-)
map.geo.admin.ch/?topic=ech&lang=en&bgLa yer=ch.swisstopo.swiss image&layers=ch.bav.ha ltestellen-oev,ch.swisstopo.swiss tlm3d-wanderwege&layers_visi bility=false,true&X=17 7923.52&Y=821085.09&zo om=11.