If you really want to visit a neighbouring country, but have limited time and funds, just hope over to Singapore.
If you have more time, go over to Sarawak on the island of Borneo (now known as Kalimantan). It is still Malaysia, but seems like a different country altogether. Don't let that Anaconda
movie worry you..... Borneo is nothing like that, although there are some jungle legends about giant lizards and strange animals living in caves.
If you have more time still, head down south to Indonesia. I took a bus from the Malaysian border town of Entikong down to Pontianak, which is located directly on the equator. I have some friends in Pontianak and you could go visit them.