In general, if you're talking about long periods, leaving plants in standing water is a bad idea because the roots rot. If you want to provide a higher level of humidity, then standing them on a tray of gravel or pebbles, and filling the tray with water to the top of the gravel, works very well.

BUT - for a holiday, ie for a shortish period, you can get away with it, and it's definitely preferable to no water!

In autumn and spring, smallish plants will cope for just over a fortnight (3 weeks is pushing it) in a greenhouse if you leave them standing in a good inch of water - it takes the first week or so to use up the water in the tray, the second week to dry out the soil, and they're ready for a good drink when you get back.

Outside, you also get rain, so you have to check the weather forecast and adjust how wet you leave the plants - you don't want them to be up to their necks in water for two weeks.

In summer, it's that much more difficult. Small plants (indeed, any plants other than cacti) are going to struggle in a greenhouse without someone coming in to water them. And there are security issues too - dead plants, or plants in the kitchen sink, are a good sign that the owners are away and the house is empty

I do a combination of things. First I try to get as much as I can of the stuff in pots planted out into the garden. The things that are going to stay in pots I deal with in two ways. Some plants I put in the shade, with or without a tray of water, depending on how well they cope with drought. Other things I bring into the house, and stand in the bath with about 3 inches of water. It's not ideal, but although more plants are killed by overwatering than underwatering, it usually takes more than a couple of weeks to kill by overwatering, whereas you can kill in a couple of days by drying out completely.

Probably the best solution is to have a reservoir of water flowing at a controlled rate on to suitable cloth on which you stand the plants.But you would want to set this system up and have several practice runs before leaving it unattended over the holiday. If you don't have a couple of months to spare, I wouldn't risk it.