particle effects, sound effects (I suggest using a teeny bit of pitch randomization so they sound less samey) camera movements (zoom in/out depending on how many enemies are on screen etc). knockback, making enemies flash or squash/stretch on hit, camera shake, etc. anything that enhances the gamefeel-- the more reactive to a players actions, using sounds or visuals, the more locked-in the player gets and the more satisfying it is to play. Tween everything you can-- instead of setting a hp bar to the new value, use a tween to animate it. when you get hit, shake the hp bar, shake the whole screen etc. it's all about enhancing what you've got happening on-screen and from the player inputs. these are just some ideas but you can find your own ways you like to juice up your games.
you guys did a great job! awesome work for a first jam for Psi. I find every jam builds skills and the next jam you can push further and further.