there lots of places to go in Malaysia, depending on what you like.
if you are a beach junkie, Langkawi Island is not bad. if you come here, make sure to book a package to Pulau Payar (Payar Island) for snorkeling trip. it's to die for!
but i prefer Perhentian Island, tho. i've also heard Redang Island is also amazing but i never been there. (hoping i'll get there someday). Both Perhentian Island & Redang Island are in Kuala Terengganu state, which is in east-coast of Peninsular Malaysia.
if you're planning to start ur journey from Singapore and heading to North Malaysia, then you should stop to Melaka (the Historical City. stay in Bandar Hilir town) and you can go anywhere on foot.
after Melaka, you can go straight to KL or stop at Negeri Sembilan. Popular beach in Negeri Sembilan is Port Dickson.
you can stop at Cameron Highland for strawberry picking if you like. it's between Perak and Pahang state..it's on your way to Northern Malaysia.
Penang (Pulau Pinang) is a good place to go too.it's kind of Melaka, but the food and the people is different. i wouldn't miss it
and there's also East Malaysia: Sabah & Sarawak too.........