I am a bit of a geeky person. I like everything related to the tech industry and work in a startup, so I am into the French tech area. Le Sentier is the street of a lot of startups, incubators and venture capital firms.
Some others are also in the street nearby and a bunch of events are organized in two bars pretty close by: The Players bar (rue Montmartre) and Le Delaville Café (Boulevard de bonne nouvelle). If no events are held there then go to the Passage des Panoramas, to eat or just go sight-seeing!
The NUMA, hosted by the Silicon Sentier association, just opened at the end of 2013. It aims to become the main place for young startups, with 6 floors to host teams and meetings.
This area is very interesting because it used to be the heart of the industrial area in Paris. Today, there are still a lot of shops specialized in the whole sale of clothes and some sell their stock (the old collection, like Kookai and NafNaf, rue Reaumur). I used to work there and when I arrived in the morning there were always people waiting for trucks full of fabrics to bring to their specialized stores and style offices.
After your walk in the area, you can stop by the rue Montorgeuil for lunch or coffee which is at just around the corner.
The mix of styles is quite interesting!