It depends on the time of year, but I like to walk to the 1st or 2nd level, then take the elevator to the top level. If it's the summer, be prepared for crowded elevators! Sunset is a great time to visit, too. Home Page
califgirl This is a fun question! I visited the Eiffel Tower many years ago, and decided to go at night. There was virtually no crowd and the lights of the city at night are truly spectacular. I'm curious to hear what other MRI have to say - especially if any have been both during the day and at night. I love marylander54's idea of sunset, as then you'd most likely get the best of both worlds. When are you going?
The only place you can buy Eiffel Tower tickets is on their website or by queueing at the entrance. No one sells discounted tickets (lest you be fooled). You can purchase as part of a tour package, but I can't think of why anyone would want to do that. It's best to just purchase tickets on the website. La billetterie officielle de la Tour Eiffel
To save time and avoid queues for other Paris monuments, check out the Paris Visite Pass. The Paris Pass® - Your Sightseeing Pass For Paris
Additional info: The Eiffel tower - tickets and practical information
And please don't miss the Sainte-Chapelle. It is one of the most glorious versions of Gothic architecture, built in only a few years (as opposed to most that took centuries), because King Louis IX wanted a giant reliquary for the Crown of Thorns, which he brought back from Crusade. It is inside the Palais de Justice, just across the Ile-de-la-Cité from Notre-Dame, and you can combine a ticket with the Conciergerie which housed Marie-Antoinette, Louis XVI, Robespierre, and others.
Whenever my students go to Paris and say they haven't seen the Sainte-Chapelle, I tell them they haven't seen Paris.