Take the train to Welshpool and either walk or cycle along National Cycle route 81, alongside the route of the Montgomeryshire canal. Stop for refreshments by leaving the route to go into Berriew village. Level walking.
Details of the walk are online here
You can also stop off at the Pwll Penarth Nature Reserve 3 miles outside Newtown, as the route passes the entrance. Sit and relax in the bird hides or see salmon jumping the weir on the river Severn (at the right time of year, of course!) See more on the Montgomeryshire Wildlife Trust website