Finished S&T internship---> How to I quantify in resume?

You've nailed exactly what makes S&T internship bullets hard to write. That said, you've also summed up all the things you can and need to write about. If you stay in S&T or finance, many people know what it's like (stools and coffee) so it's not a big deal if it feels light.

I recommend the below structure for S&T interns; a summary of your desks and 2-3 bullets on your projects. I imagine the extent of your "projects" can be summed up with trade ideas, excel database clean ups, and some independent research.

The 2-3 project bullets should be written like any other bullets with ideally 3 but at least 2 parts: 1) what you did, 2) why you did it, and 3) what it resulted in (will be hardest to quantify as an intern).

• Rotated across [x-sales], [y-trading], and z desks and interacted daily with sales, trading, and research teams.
• Developed [automated process] to look at [xyz desk's biggest clients by trade volume] resulting in [increased contact with 5 large accounts].
• Analyzed / researched [historical trends in abc market] to understand and anticipate [possible effects of a Fed rate hike].
• Researched and presented trade idea pitch on [stock / spread trade / etc] that focused on [xyz as a catalyst] and the [mean reversion in short term spreads].