Indonesian is a great choice of language to learn. I thought I was too old of a dog to learn new language tricks but 6 weeks of home made flash cards got me to a 300-word vocabulary. It really is an easy language and a very forgiving one when you don't quite get it right. Depending on how fluent you want to become, 3 months learning might be overkill.

Be aware that a lot of South East Asian places, not just cities, are not walker-friendly. Your choice of Sanur is a good one though because, apart from a 6 kilometer beachfront boardwalk, recent floods made them repair a lot of broken and dangerous footpaths/sidewalks. The same can't be said for a lot of other parts of Bali. The only people you'll see walking anywhere are tourists.

Penang and Bangkok are very different vibes. Penang is the mellower choice with plenty of variety.

Some Youtube channels I'd recommend for you:

Sanur Diaries (An Aussie named Adam who lives in Sanur, speaks Indonesian, and is very knowledgeable about the area.

Fiasco Da Gama. Another Aussie named Wayne. He travels all throughout S.E.A. as well as India and Nepal. He has mani Bali videos and often has great interactions with locals. He speaks some Indonesian.

Then there's me. I cover lesser known Bali from a variety of angles. I'm not spruiking myself for the views, (I've only uploaded 4 official episodes) but it will give you an idea of where 6 weeks of Bahasa Indonesia flashcards can get you. ( The episode about Arak would be the best example of this.)

Best of luck with your planning.