There are two queues to enter the museum. Most people go in from the ground floor, entering through the glass pyramid. To avoid the long queue, I would suggest taking the entrance from the basement. It's linked to the metro and I queued for only 5 minutes or so to enter the museum.
There is free admission for 18 years and below and also on certain Sundays. Check the admission details before going! I managed to get in for free :)
There were just too many places to cover in beautiful Paris. Some of the places we missed were the Musee d'Orsay, Luxembourg Gardens, and the Palais Garnier.
The Sacre Coeur can be reached by the metro, stopping at Anvers. However, Montmartre is known to be a pickpocket's heaven so it is always important to be vigilant. Along the way to Sacre Coeur, there are plenty of souvenir stores and they are pretty affordable.
Overall, my favourite place has to be Montmartre because of the artsy vibe it gives. It's a really unique experience which you don't see in Singapore.