Welcome to the forum
The Switzerland Mobility Website can give you some ideas, for a start :
www.wanderland.ch/en/h iking-in.html?phrase=Spiez+%28BE%29& sname=&sid=439
March is still Winter at the higher elevations of the Bernese Oberland , such as the Jungfrau region.
Weather conditions are highly dependent on altitude. www.myswissalps.com/ weather
In the warmer and sunnier lower areas, there will be distinct evidence of Spring, but it will be a few more weeks before the trees begin to flower. I'm thinking of the Emmental countryside between Bern and Luzern, or Luzern. The earliest warm days tend to be on the west and south facing mountain slopes....Montreux,Lausanne, Weggis on Lake Luzern.
The south facing wall of the Rhone Valley is warm and sunny, hot and dry in the Summer, easily accessible from Spiez, has a super hiking trail paralleling the rail line. The so-called BLS Südrampe trail usually opens officially at about Easter. Before that time it is officially closed, but some people hike it with care. Opening date depends on the trail conditions, and the conditions on the mountain walls above the trail.
I have walked it about a week before the opening while the winter debris and rockfalls were being cleared. It may be blocked, or may be passable with care. Depends a lot on how the Winter weather was, and how much snow fell on the mountain slopes above the trail. One picture shows the handrail and cables that will be installed along the outer edge of the walkway in a few days after the picture was taken. The beer garden above Raron was open then, and there were a very few hikes on the trail. You can reach it by auto, too.I usually walk from Hohtenn to Ausserberg ...Both have train stations. You can continue all the way to Brig.
www.wanderland.ch/en/h iking-in.html?phrase=Station+Hohtenn +%28VS%29+-+Hohtenn&sname=&sid=10 3860
The last link shows all of the trail. the section from Hohtenn eastward is the most commonly hiked portion, because of easy access to the trail at that station.
The rail line is very scenic between Spiez and Brig on the upper route via Kandersteg;
Google search for Lötschberger and look at the various listings.
Attached are some photos from along the hiking trail from the week before the official opening.
Note the signpost at the beginning of the walk at Hohtenn Station has a red sign saying that the trail is not open. Travel at your own risk, wear sturdy hiking shoes or boots until the trail is fully cleaned up. Ask about it at the tourist office in Spiez.