Cruises
There are several boat operators who offer day cruises. Most guided tours from Te Anau or Queenstown arrive in Milford around noon, so it is advisable to escape the crowds by going on an early-morning or late-afternoon cruise. If you intend to take photographs, the quality of light is usually better around those times as well.
There is also the option of an extended cruise on Milford Sound - overnight cruises operated by Real Journeys). See "Sleep" section below.
Milford Deep Underwater Observatory
(http://www.milforddeep.co.nz/). this place can only be reached via one of the cruises. what they are trying to do is interesting, but they charge a lot to see the place and you get a whopping 15 mins total there. there is not even enough time to look through the displays.
One of the most awe inspiring ways to see Milford Sound, is from the water level itself. There are two ways to do this:
kayaking
Your proximity to the water in a kayak just serves to make you feel even smaller, and makes the Sound seem even more vast! The pros of doing it this way are that you get to go places that the big cruise ships cannot, you see a great deal more wildlife penguins, seals etc. as you move more quietly through the water, you are part of a small group between 6 and 8 and so getting more personal attention from your guide, and you get some exercise in the process. Cons are that it is quite hard work and you should be prepared to get cold and wet. A good way of doing this kind of trip is on a 'one day package'. You can be picked up from your accommodations early in the morning around 6:30AM in Te Anau by minibus and driven to Milford, where you are kitted out with all the relevant kayak and safety gear and given waterproof bags to take cameras and food with you. The trip lasts till the late afternoon lunch is taken in your kayaks in the middle of the Sound and you will be driven back for around 6PM.