Milford Sound is a must-see for anyone visiting New Zealand. The spectacular scenery at the Sound is very hard to beat (despite New Zealand offering some of the most fantastic views in the world). The five hour drive to/from Milford Sound isn't bad either. Various tour operators travel between the 'adventure capital' of New Zealand and the Sound, stopping at Te Anau (the gateway to Milford Sound) en route.
Depending on your budget, and comfort requirements, there are various options for your coach operator. You can choose whether to start your journey in Queenstown (with most of the other passengers) or start from Te Anau. Most of the tour packages available will include a Sound cruise once you reach Milford. The size and type of boat at the other end will vary depending on operator, although most are very comfortable and not too overcrowded. We travelled with Kiwi Discovery, which provided an excellent service (our driver was called Bruce - very funny) including lunch on-board the boat at Milford Sound.
Most of the operators will give you the option of upgrading to a flight for the journey home, rather than travelling by coach all the way back. This can be a great addition to a memorable day out, but bear in mind that this optional extra can be extremely expensive, and a premium can be added if you book on the day rather than in advance.