Hey there,
March's the end of summer and autumn arrives in April. It can get a lil chilly but not too cold, which is nice. Spring's in October and it's always lovely. Melbourne's known for its unpredictable weather though, having 'four seasons in a day', so it's always good to be prepared.
What sorta stuff you're into? If it's nature, I nominate the top 3 spots:
1. The Great Ocean Road
2. Wilsons Promontory
3. The Grampians
The Great Ocean Road is a beautiful scenic coastal drive and it takes about 3 hrs to get to The Twelve Apostles, which is a must-see for any visitor in Victoria. There're also surf towns along the way with great beaches.
The Otway National Park is a nice place to explore the rainforest and camp out. Occasionally, some lil treks could lead to a few hidden gems, all the way out to the coast with surreal-looking, deserted beaches.
Wilsons Promontory is a much less visited place and it's at the most southern tip of Victoria. It's got beautiful beaches to chill out on and mountains to climb. I went there in winter so the weather was kinda mean but we still managed to trek up and enjoyed stunning views of the coast. I suggest at least a couple of days to explore the national park.
I haven't been to The Grampians but I've heard so much about it. Lotsa great hiking and spectacular waterfalls. Then there's Mornington Peninsula and the National Park itself. I saw the most beautiful sunset ever from the coastal cliffs at one of the rugged back beaches...have seen countless sunsets but that one just totally took my breath away.
Anyway, there's just so much to do and see. It all comes down to whatcha wanna do on your trip.
It's really easy to drive around in Victoria. The Melway is the driver's bible, which covers the region with excellent detailed maps.
The official Tourism Victoria website is an excellent source for information.
http://www.visitvictoria.com
Hope this information helps.
Cheers,
Geraldine