Leaving history, churches, art etc and immerse into nature-where to go?

I would head to the gorgeous Dolomites for some hiking. Probably going from Venice will take less time.

Late April may not be a good time to visit the Dolomites - depends on the weather conditions. Trails may be too muddy from spring thaws to make hiking possible.

My first thought was the Dolomites. but you might want to consider Lake Garda as well. Not too far of a detour when traveling between Venice and Florence.

But then, like the Dolomites, Lake Garda doesn't have swimming weather in April. I checked and the average daytime temperature is in the high 50's.

Tuscany. There are beautiful hikes around San Gimignano and Volterra, as well as the Chianti region. Easy to get to by train or bus from Florence.

Abruzzo has a lot of great places to hike. Much green space

I thought the Po Delta is in the right location, but no car may make it harder as I don't think there is a train. It's along the coast between Chioggia and Comacchio. Late April is great Spring bird migration time.

It can be pretty difficult to get into nature without a car. One possibility might be train to Florence, train to Lucca, bus to Barga. You can access some hikes from the town. This page might help: http://www.50hikesoftuscany.com/
I'd delve further into the Dolomite idea, as I don't know that area as well. Weathger is so wacky these days, who knows what spring might bring.

Thank you for your travel ideas. I am looking into them all in order to determine what works for us. Happy Travels All!

The easiest alternative would be the Dolomites north of Venice.

I have fond memories of the Po Delta Park, where I had the best eel sandwich of my life. It seems you can sleep there now. You could pick a farm-with-rooms and see if they can organise a transfer from the closest station. As a side note, It's a delta: Always ask about mosquitos.

I wonder if you could get to the Maremma Natural reserve south of Florence without a car. There are trains to Grosseto. It may be Worth googling a little how to get from Grosseto to Marina di Alberese.

For sure it's closer to Florence than The Abruzzo National Park, but smaller.

I will be traveling to Italy the end of April

In case you mean April 2023, note that both April 25 and May 1 are national holydays.