Shouldnt need to do any cleaning. I would probably empty whatever whiskey (preferably into a growler or other drinking vessel) is left in there and then fill her up! (Make sure all the staves still look wet and sealed from the inside)
Couple items to note, throw an airlock on there for the first week or so. You might kickstart a little residual fermentation. If you have a solid bung on there with will become a flying object...
One thing I've learned with the smaller barrels is that there is more surface area contact with the barrel staves per gallon than with traditional 53 gallon barrels, so it will definitely take less time to get to that "perfect" barrel aged point than would at a commercial brewery. I found that any 5 gallon barrel aging I've done, they are usually ready within 2-4 months compared to the 12 month+ aged beers you would get from a commercial outfit.
Also, typically the barrels arent a one time use thing. Eventually you will start to lose the oakey and whiskey flavor, but with my barrels I got 2-4 uses out of them. I would just fill them back up with a bottle of whiskey after barrel aging and rotate it to keep the staves wet.