Based on my 7-day family trip, I spent USD $20 total in cash. For shorter trips, budget US$5/day cash in Malaysia, or about 20 Malaysian Ringgit (MYR). Cash was used for hotel or guide tipping, toilets, and the occasional cash-only ticket. I'm not going to list all the tourist attractions, but a few popular ones that you may visit. Malaysia is actively encourage digital payments and you will see many signs posted everywhere in Kuala Lumpur about digitization.

Kuala Lumpur

  • Batu Caves is a temple and free to visit. You may want to bring cash if you want to make a donation or want to go to areas of the temple to worship. When you enter the area by the car drop-off there will be some locals wearing some "staff" t-shirts literally herding you to the Batu Cave Snow House which is a paid tourist-trap. Did you really think they built an indoor snow/ice cave 100 years ago?
  • National Textile Museum (the website only supports HTTP and not HTTPS) is cash-only.
Non-Malaysian Price
Adult (13 years old or more) MYR 5
Child (6 to 12 years old) MYR 2
Child under 6 years old Free

Penang

  • Kek Lok Si Temple is free to visit, but you may want to bring cash if you want to make a donation and pay for the air-conditioned funicular ride to get to the top. One-way is RM8 per adult and RM4 per child (7 to 12 years old). Double those amounts for round-trip. Expect to pay round-trip for the funicular as it's unlikely you will hike in the hot temperature to the top.
  • Pinang Peranakan Mansion takes credit cards.

Langkawi

Cash-only for some local shops and perhaps for massage (the legit kind). I mention this as some massage places have huge signs saying "no sexual services" in case some customers think it is a "special massage" type of place for some reason.

  • Kilim Geoforest Park - While I paid online for a tour in advance, I decided to tip the boat driver. I also tipped the driver who drove us to/from the hotel, although they were surprised to receive a tip.
  • Bat Cave in the park - budget MYR 2 per person. While Child tickets cost less and depend on age, do you really care about up to MYR 2 (US$0.25) difference?
  • Toilets - 1 MYR per person. This could be in small shopping malls or even at the Kilim Geoforest Park.