I find doing the motions with no weights at all is actually challenging. I usually find at a certain point there's a click where the downward force of the weight is enough to actually force me to use the right form. Now given that a bar without weights is 45lbs, it's probably more accurate to describe it as doing the form without weight plates, rather than doing the form without weights entirely.
With dumbbells specifically, you could use them to practice deadlifts as you would do them with a trap/hex/shrug bar. Naturally you'd have to place them on a box that puts them at the right height. The deadlift is the exercise I found I had the least difficulty getting with good form right off the bat using minimal weight (possibly because lifting a dead weight off the ground is something I've been doing almost all my life). Otherwise putting a broom stick, or some other stick on the boxes, getting it right against your shins could work, although you'd probably want to off centre it so the broom part doesn't unbalance the stick.
The press should also work well, particularly since it is not unlike stretching your arms up above your head. Also, like the deadlift, it's a motion you've likely done quite often to put something on a high shelf. The dumbbells wouldn't work quite so well as a barbell substitute as it's pretty hard to hold them the way you should hold a barbell in front of your chest for the press.