Caloric Vestibular Stimulation seems very promising: Cholinergic system changes in Parkinson’s disease: emerging therapeutic approaches ncbi.nlm.nih.gov/pmc/articl...
"Another neuromodulatory approach that could improve gait and balance in Parkinson’s disease through enhancement of pedunculopontine nucleus connectivity is caloric vestibular stimulation.94,95 In a randomised double-blind, placebo-controlled study, 33 patients with Parkinson’s disease completed 8 weeks of treatment at home. Patients who received active stimulation had significant improvements in both non-motor (especially cognitive) and motor symptom burden scores compared with patients who received sham stimulation."