What should I do to make good on some lightly water damaged plaster?
If the surface is undamaged and it sounds solid just leave it to dry out completely. If it's now dry to the touch then it is drying out. If there was a lot of water coming in it will take a while to dry out.

Once it has dried then seal it with a coat of thinned white emulsion before putting the main colour on or wallpapering.

One caveat though - it could be permanently stained so it might still look damp but be perfectly dry - so even if the discolouration hasn't changed in week (or so) it will be safe to decorate. What you could do in the first instance is just paint. If there is still a problem with the damp it will quickly show up and you can then look at replastering.