What to wear to work?

[Fillingthepiggy]

30, 5ft2, size 8, small chest.
Work in a pretty casual office where pretty much anything goes (with the exception of client meetings when you are obviously expected to smarten up a little bit) but I’m stuck in a style rut! I’m a jeans, T shirts and converse kind of women, don’t really do heels (commuter- have 20 min walk to the station and once in London have a 20 min walk to the office)
Any suggestions on how to be a bit more “grown up” for day to day and a bit smarter on the odd occasion?

Could you wear trainers for your commute and keep a pair of heels/shoes under your desk to change into?
Switch jeans for cigarette pants or chinos?
Wear a shirt instead of a t-shirt. Roll the sleeves to stop it feeling too formal.