In MIL's kitchen, there are varnished wooden units. About 20 years old.

The varnish could do with some livening up, generally. Also, around the
sink, there have been some water splashes that have left lighter patches.

The plan is to do the best I can in say an hour, ie I am not going to
try stripping it or anything like that. I was thinking of just going
over it with wax.

Could the clever people here guide me what type of wax is best for this?
Or should I use something different?

Also, should I go for clear wax, or wax about the same colour as what's
there, or go for something a bit darker to cover up the water damage?

For polishing it, can I just stick a duster over an orbital sander?