Islands
Redang - One of the most popular islands, more couples/family than backpackers. Beautiful and usual beachy stuff, diving etc
Tioman - We went here recently. Amazing place, much smaller and good vibe - a bit more backpacky. It's duty free so booze is cheap, and all restaurants are BYO as they're muslim but let you drink so you just order mixers. Hire a rod and if you catch a fish any restaurant will cook it for you whilst you sit on the beach. If you go do the boat tour with a stop at the waterfall, good but short walk then a swim in the waterfall
Rawa - Not been but friends raved. Again much smaller and more romantic (our friends went for their anniversary)
Kuala Lumpur
KL is the only city but there isn't a huge amount to do, 2/3 days at most depending on how quickly you like to travel.
Jalan Alor - Must go for dinner. Just a street full of food vendors and local restaurants. Try some durian from a stall if you dare.
Petronas Towers, Central Market, China Town (Petaling Street) and Bird Park - All on the tourist trail but worth a visit if you think they're your thing
Batu Caves - Worth a visit, it's a short taxi ride out the city. Big cave complex with temple in, can either just visit or book a caving trip (I think)
Taman Nagara
Their biggest rainforest. Stay at Mutiara Taman Negara which is the only resort-esque place there and is actually in the rainforest. All other accommodation is across the river and very backpacky/hostely. You get there via boat which is awesome in itself and you can then do rafting, walks through the jungle and a few other expeditions which you can just book onto whilst there. Two days there is enough but worth it.
Others
Cameron Highlands - Tea country. Nice and pleasant and a break from the hot sticky weather - it's a good 5-10c cooler there. Quite nice to drink tea in a plantation but otherwise not a lot there so one night is fine.
Malacca - Not been since I was younger but basically a colonial area/city to the south of KL
Singapore
Worth a visit I think, it's not as "bad" as Tom is imagining. Much more cosmopolitan/compact and only clean by Asian standards. If you go then check out Little India, Marina Bay Sands (rooftop bar), Singapore Zoo (where you can meet orangutans) and Chinatown.
If you want to fit a lot in then hiring a car may be useful but public transport isn't that bad if you can get your head around timetables and the like. We went from Tioman to KL, Taman Nagara, Cameron Highlands and all the way back to Singapore on coaches and it was fine. Driving could be way more stressful and you'll need to find parking and get on boats for the rainforest and islands anyway.
Don't want to employ myself as your tour guide but based on where places are geographically and what they involve doing I would do KL -> Taman Nagara -> Cameron Higlands -> Redang -> KL -> Tioman (ferry is from Mersing so that'll be the coach destination) -> Singapore -> Malacca -> KL.
