The mid-length treks in Nepal on this website offer a range of challenges and are ordered by difficulty. Panch Pokhari is relatively ‘easy’, Rara Lake, Helambu-Gosaikunda-Langtang, Annapurna Sanctuary/Base Camp, Everest Base Camp, the Annapurna Circuit and Upper Mustang are a little more strenuous, but not difficult. Panch Pokhari is a camping only trek, the rest can be done as tea-house treks, ie with accommodation provided, though the accommodation is basic on the Rara Lake trek.
Bear in mind that all the treks involve hill walking, if you’re only used to walking on the flat then none will be ‘easy’. They are listed with the ‘easiest’ at the top.