Indoor vibe: The ULIP library is good if you get easily distracted and/or like to work in silence and has windows which provide natural light unlike in the basement! However the ULIP basement is good for those who prefer to work in groups, discuss projects, be near the kitchen and have regular snacks (as there is no eating allowed in the library, however drinks are fine). Also figure out what time of day you prefer working. The quiet times around the university are usually first thing in the morning or at the end of the day.
Change of scenery: Of course you may prefer to have a change of scenery either at home or somewhere near university if you have a few hours break between lectures in which case there is the American Library of Paris. ULIP has a partnership with this library so you can pay them a visit and set up a student membership. It will only take 5 minute to set up a membership, remember to bring your ULIP card as proof (https://americanlibraryinparis.org/). This library is particularly popular amongst ULIP students as they are open on Sundays from 1pm. The ULIP building is shut on Sundays.
Café vibe: If you prefer a French café vibe whilst studying and can treat yourself to a drink for 3 or 4 euros, check out this cute café, located by Ecole Militaire Metro line 8, that has guaranteed internet connection: Kozy Café ( http://www.kozy.fr/ ). If a café is your preferred study environment, you will probably find your own local favourites, but this is a good starting point. Equally, you can try out Café Nuage, located by Maubert Mutualité Metro line 10, a co-working coffee shop where you pay per hour instead of per food/drink item. Perfect if you're a coffee and madeleine addict because this means unlimited snacks!
Outdoor vibe: When the weather is nice, I would also advise taking advantage of the esplanade outside university for reading/revision/discussing group projects or for study breaks.