The Swiss public transport system makes a car genuinely optional here. The SBB and Thurbo network reaches every town of any size, with timed connections at Romanshorn, Weinfelden, and Frauenfeld. The Bodensee Ticket day pass covers buses, trains, and ferries on both sides of the lake for around CHF 24 and is the best value for active sightseeing.
For the rural castles and the inland orchards, hiring a car or, in summer, an e-bike is more efficient. The terrain is gently rolling — cycling is excellent and the canton has an unusually dense network of marked bike routes (the Bodensee-Radweg, the Apfelweg, the Mostindien-Route).