I keep looking at different sites trying to figure out what island to visit in Hawaii. I'm planning on going for a week in April and would love to do typical Hawaii stuff...waterfalls, volcanoes, snorkling but I'm having a really hard time figuring out which is the best place to go. Thanks for your help!

No offense, but if you want more responses, you might want to repost with a broader headline that puts Hawaii in the topic.

I've spent time in Kauai and Oahu. The former is nice if you want a low-key environment with not much in the way of night life. Oahu is probably to metropolitan to give a true flavor of Hawaii.

Why settle for only one island?
For first timer would recommend:

#1 Maui
#2 Kauai
#3 Big Island

Feel free to island hop between all three!

Oops, missed the one week part, suggest 4 days Maui, 3 days Kauai.

The true problem lies in the fact that each of the major islands is unique and has many different things to offer. I have been to all the islands and go back on fairly regular basis. As I plan my trips, I still find myself thinking "what will I miss out on Kauai or Maui this time, if I go to BI" or whatever. When friends etc ask me this question, I usually refer them to Maui as a starting point. It has a nice balance of shopping, nightlife, outdoor activities, beaches and great dining. If you are more of a laid back, relaxed person where nightlife and shopping are not so important, the I would go for Kauai. The only thing you are going to miss out on is the active volcano aspect on the Big Island. If this is a major priority, then start there. I guess my point is that you cannot really see and do it all in one week, or even a few. Do a quality visit to one and plan on going back if you like it! Have fun.

I got to Hawaii to totally unwind and I am drawn to the more remote locales. Kauai is where I'd go. I'd skip Maui, unless you go to Hana. Have a great trip.

Alohamom has it exactly right -- every island has its own charm. For the best "get away from it all" experience Kauai would be the right choice, although there's very little in the way of nightlife. Maui is sort of a "middle road" -- more developed (and expensive), but it also has some of the best beaches and resorts in the entire state. Of all the islands the BI is my favorite, only because it's so huge, has resorts that outclass any found on Maui or Kauai, and has an incredible array of natural environments and activities. One could spend months there and still not experience all of the towns, geology, activities etc. Also, it's the only Hawaiian island where you can see a live volcano.

But as others have said, for a 1-week trip you should focus on quality instead of quantity. For the activities you mentioned (except the volcanoes) Kauai, Maui or the BI will work just fine.

While I agree that each island has its own charm, if you are travelling a great distance (e.g. 10+ hours of flightime) and don't get to Hawaii often (does any mainlander get there often enough?) it makes sense to do more than one island, because natural conditions or quality (e.g. drought, beach erosion, wave action, lava flows) vary among the Hawaiian islands.

For example, on our first trip to Hana (2001) the waterfalls were amazing, while on our last visit (2003) Hana was parched and not very impressive. Also, on our last visit to the Big Island, the volcano (Pele) was putting on a show according to the rangers, whereas recently I have heard the activity is less and more distant.
Even on Kauai, I have heard that the stunning Napali coast trail can wash out.

By covering more than one island I think you are more likely to discover remarkable experiences, though you do have to pay a price in terms of airfare and interisland travel time. Just my 02 $)