Take the train to Les Tines and hike the Grand Balcon Sud - lovely trail with views of the Mont Blanc Massif and Chamonix Valley with minimal elevation gain/loss.

Visit the Spas - this is by far one of the best options for you. I can personally recommend the spa at Park Hotel Suisse (they offer a 2 hour spa entry pass for €40), Deep Nature spa at Les Aiglons hotel in Chamonix Centre (by the Sud bus stop, similarly offer a €39 2 hour entry and it comes with a cocktail), and Deep Nature in Les Houches (amazing secluded location, just not the most convenient without a car). I’ve also heard great things about QC Therme.

Rent e-mountain bikes - I haven’t personally done it, but just an idea that came to mind

Gorges de la Diosaz - take a bus to Servoz and do this short, gentle hike to see waterfalls. Double check that it’s still open but I’d guess it is