Well, it depends.
And that is the simple answer. The answer is going to be more complex depending on the actual devices.
What you need to know is are the devices capable of running under both US and European electrical currents. You can look at the power adapters or the devices themselves to answer that.
Does it say 110-230 v (for volts) 50-60Hz on them, or something close to that? Most cell phone chargers are and have been dual voltage forever so you can look at one of those chargers to get an idea of what you are looking for. If it does, you can get by with just a plug adapter. You can buy those right here in the RS online store for $1 each.
If the devices say 120V 60Hz (or close to that) and nothing more, you will need a converter to run them. Those are heavy, expensive, limited in how many devices are simultaneously able to be plugged in, and no fun to drag around.
Until we all know these details, we can't really recommend.