Which island, and please do not try to do four
This is the decision the whole trip turns on, and the usual mistake is treating Hawaii like one place. Each island is a separate flight, a separate rental car, a separate hotel check-in, and a half day gone each time you move.
Oahu if it is your first time and you want a bit of everything. Waikiki, Pearl Harbor, the North Shore, real restaurants and nightlife, and the easiest logistics because everything else connects through Honolulu.
Maui if you want the classic Hawaii postcard. Resorts, beaches, the road to Hana, whales in season. It is the most popular island with visitors and it prices accordingly.
Kauai if you want green and quiet. The Napali coast, canyons, waterfalls, and far less development. Bring a sense of adventure and rain gear.
The Big Island if you want strange and dramatic. Active volcano, black sand, snow at the summit and lava fields that look like another planet. It is the largest by a distance, so the driving is real.
Two islands in ten nights is comfortable. Three is possible in two weeks if you accept losing a day to each move. Four is a travel itinerary, not a vacation.
When to go
Hawaii is good year round, which is why nobody warns you about the pricing. The expensive weeks are mid-December through early January, spring break and most of the summer. The value windows are late April into early June, and September into early December.
Whale season runs roughly December through April and it is genuinely worth planning around if that matters to you. Surf on the north shores is huge in winter and calm in summer, which flips which beaches are swimmable depending on when you come.
The things that catch first-timers
Jet lag going home is the bad direction. Flying west is easy. Coming back east from Hawaii is a long night and a rough Monday. Do not schedule anything important the day after you land.
Resort fees and parking are real money. Many resorts add a daily fee plus paid parking on top of the room rate, and it is not always obvious at booking. I price the actual total so you are comparing like with like.
A rental car is essential on most islands and a nuisance on Oahu. In Waikiki you will pay to park a car you barely use. Elsewhere you cannot see anything without one.
Sunrise at Haleakala requires a reservation booked ahead, and people find that out the night before. Same story for Pearl Harbor tickets and the popular Napali boat trips in season.
Book flights before hotels. Hawaii flights from Texas are the constraint, not the rooms, and the good fares set the shape of the whole trip.