It’s been a few years since I’ve been to Legoland but I used to be a regular with my DC. The first 90 minutes of the park opening is the least busiest so head immediately to the rides you want to do. Save the mini LEGO village and the Star Wars LEGO themed area and the shops etc for later in the day when the park is really busy.
The food is terrible and expensive, take a picnic. My DC loved the pirate show so we would head there 20 minutes before the show to secure a spot and eat lunch. The puppet theatre is also great and a good rest spot for tired legs.
Study a map of the park and plan your day, Legoland gets so busy, by the middle of the day it’s like the 7th circle of hell. You need to roughly know what rides you want to do and where they are located and do them first thing.
The last hour before the park closes is also slightly quieter. So any rides you don’t manage in the first 90 minutes, plan for the last hour.
The 3D cinema (films are 10 minutes long) is also well worth going to, we would attend towards the end of the day on the way back up the hill to the car park.
Take swim stuff for the splash park. Just double check the website for opening times.
Download the Legoland Windsor app, this has a map of the park, times of shows, details of any closed rides and rough waiting times on the day.
Lastly, the car park used to open an hour before the park opened, I would advise getting there as soon as possible; the car park is vast and it can take several minutes to walk from the car park to the entrance.
Have a great time!