Math Circles are one type of "workshop" I'd recommend. Math Circles can take many forms: More students or fewer, more lecture or less. But the idea is that the students do most of the work, and the leaders are there to give helpful nudges. (Perhaps 10 students with one leader, 15 with two, 7 or 8 for each leader if more.)
Good problems are "low floor, high ceiling", easy to get started, and full of rich content. They also can be approached from many directions. You need problems that will intrigue students. Resources include: Tom Davis's page, mathcircles site, National Association of Math Circles, and a few posts on my blog, including this one about doing a math circle with the game Spot It. There are no important topics, only good ones and not so good.
Instead of having all participants working on one problem, you can have a festival of mathematics, like the Julia Robinson Mathematics Festival, with a different problem sheet or puzzle collection at each table (along with a table leader).
Since most math leaders have not been trained in this style of work, it's important that the leaders get experience playing with problems together before leading.
(I see that you are in India. I just now googled "math circles India" and I see a number of promising looking sites. Maybe the people behind those will have some advice for you too.)