On my first visit to the Grampians, I travelled with VLine from Melbourne and predominately visited Halls Gap (the only town in the Grampians) on foot. Not having a car had its limitations and in hindsight it would not have been possible to walk to the top of Mackenzie Falls, an Australian kindly gave me a lift. Although there were numerous cool places to see which you could realistically walk to from the hostels in the town centre.
Swimming is one of my hobbies, so as well as Mackenzie Falls I also went to the Venus Baths and Lake Bellfield. I found my dip in the Venus Baths and Mackenzie Falls very refreshing. Sadly though, Lake Bellfield was very dirty and despite being over 6 foot it felt uncomfortably deep. Perhaps the water quality may have now improved.
My ascent upto the Pinacles was really enjoyable, it involved going past the Venus Baths and Splitter Falls. My plan to descend in the opposite direction failed. Either the route was poorly marked or I missed some signs. After finding myself alone and clearly not on any sort of path I decided to return to the top of the Pinacles and go down the way I went up. On my ascent there were lots of walkers, so had anything bad happened there would have been someone there to help me. The lack of fellow walkers on my failed discent was very discomforting. If you are doing walks of this nature on your own, either stick to the touristy paths or ensure your phone is fully charged and you have signal.
Ideally I would have gone with a group but getting people in my hostel to commit proved challenging. After my first trip, I discovered numerous Facebook pages where people are offering and asking for lifts to destinations all over Australia. These Facebook pages are a valuable resource for travellers. The Grampians is between Melbourne and Adelaide, I saw a few people discussing driving to Adelaide via the Grampians. For people who want to see the Grampians but stay in Victoria, you could get a ride share from Melbourne to the Grampians and then use the VLine to return to Melbourne.
The second time I went to the Grampians was on the road trip to Perth. We went to Boroka Lookout, Mackenzie Falls and Beehive Falls. Rather than swimming at the base of Mackenzie Falls, we wandered down stream. The waterfalls were less powerful downstream so you were able to sit in them rather than just close to them. It is not possible to see Beehive Falls without a car. It was a quick 20-30 minutes easy walk from the road side. Beehive Falls offers less spectacular views than other parts of the Grampians but never the less in my eyes it was still beautiful.
My conclusion from doing both the trips is it is probably daft do attempt the trip on foot. Although even if you do have a car I would recommend you spend at least two days in the national park. You will not fully appreciate the national park if all you do is simply drive to a view point, look at the view and then repeat the cycle.