They can actually do a significant amount, like force a recorded vote. That means that everyone has to stand by and admit, on the record, what they're voting for, which becomes voting fodder later. It also significantly slows down the passage of bad bills and allows them to potentially be tied up.

They can also make frequent quorum calls, which also allow things to be tied up. Remember, these people are not working 40 hours a week, 50 weeks a year- if you stall things enough they fall off the agenda entirely and suddenly republicans have to start prioritizing what shit they're trying to ram down our throats.

Additionally they can not do repugnant shit like agree to a unanimous consent time agreement, which speeds republicans atrocities through the senate- which Democrats literally just now did to fast track Gabbard through confirmation as the Director of National Intelligence.

The minority party can ABSOLUTELY gum things up- there's all KINDS of dilatory motions the minority party can do, but right now democrats seem wholly uninterested in actually being an opposition party. In fact, top Democrats are actually mad that progressives are calling for them to be an opposition group in a meeting on tuesday!