i dont know how strong you are but even with just one dumbell up to 20kgs you might want to try overhead dumbell squats, waiters walks, push press/jerks, bent over rows (balance the shoulder muslces). be careful with anything overhead though, theres raging debate about it with very good arguments for and against...

another good one (stolen off AndrewS) is if youve got a stability ball use that as a bench and do single arm db bench press, hits core strength a lot. be careful you dont smash ya face in tho

also all the usual shoulder stuff like front and lateral raises, L-lateral raises, 3 point raises etc db swings are really good but be careful of weights flying off etc... a good place to check for pure dumbell exercises would be ross enamait's site, cant remember the link right now but i think its rossboxing.com or warrior something.