Unless you're ready to buy serious dumbbells, you won't find an exercise that would make holding a couple of extra 1-4kg toys provide growth stimuli anyhow comparable to your previous gym workouts. Your legs are already strong enough to lift 60-90kg of the body for many reps — extra little is only better than nothing, it does not really count as lifting weights and can not provide same muscle growth.
Today one can buy dumbbells adjustable up to 40-50kg each, and perform deadlifts which are similar to trap-bar lifts. Those will allow to maintain the principle of progressive overload for many months while staying in rep range that is still sensible for building muscle (less than 20-25 repetitions in one set). Using only one leg will extend that for even longer, so one pair of such dumbbells may serve an average-weight male for a year of simple lifts.
It is also possible to perform Snatch, Clean and Jerk with dumbbells. Explosiveness makes each kg count as more than two, and makes exercise overall harder and lot more exhaustive. 20 reps of 80kg clean&jerk is challenging goal which will take average-weight person few years of consistent effort to accomplish.