I'm from Malaysia originally. Family from Penang although I grew up in KL, so both are places I know extremely well. Others have already given you lots of excellent ideas for destinations.
If you don't want to hire a car (fair enough, even I find Malaysian driving scary), there are many options:
Within most towns/cities, we all use the Grab app (equivalent to Uber) to get around - very convenient and inexpensive, you can link the app to a pre-loaded credit card or a PayPal account if you want to avoid scrabbling for cash after every ride.
In KL, the mass transit options (LRT, MRT, monorail) are plentiful, cheap, air-conditioned, clean and easy to navigate.
In other cities e.g. Georgetown, public buses are also cheap and usually quite easy with the aid of Google Maps.
Long distance, for major destinations I prefer the trains, but most Malaysians love long-distance coaches which literally go everywhere and are super-cheap.
Another excellent option which is a bit more pricey is to hire a driver to take you around, there are lots if you Google or look on Facebook and they generally have comfortable, large, modern and air-conditioned vehicles - you can hire them for a specific journey (local or long-distance), or for a period of time.*
BTW, March/April is still monsoon season on the East Coast of Peninsular Malaysia.
*Anecdote: A few years ago, I met a lady who had just come back from a group holiday in Malaysia. She told me they had hired a people-carrier with a driver, all was excellent and they were very happy with the service. She said her only complaint was that it was very hot in the car without air-conditioning. My face was all
because I honestly haven't encountered any kind of passenger vehicle in Malaysia without air-conditioning for about 4 decades. Afterwards, DH and I were speculating that the driver must have specially unearthed an antique van to give his guests a more "vintage" experience 