My thoughts based on having used most of your options:
Mobile CB. Not sure, they are expensive, highly accesable. - very open channels and subject to interference. Handsets not that cheap.
Camping pmr446, - cheap, accessable, not sure about quality of transmit. - got some Motorola ones that work ok with line of sight - maybe half a mile. use them for pigeon shooting and very useful. Can still get channel interference but far less that CB.
Digital pmr. Alittle more expensive, clear audio.- certainly a step up on the camping ones - better range and usually a better power output. More expensive.
Baofeng pmr446 - cheap and analogue, aparently really good. - certainly worth a punt for the money but might be similar to the camping 446 quality
messenger Pigeons, clean up, and have to feed them. - better on the BBQ
Walkie-Talkie type radios can be good fun to use as well and you can get some chat and banter going.
Another option with mobile phones is to set up a WhatsApp group and you all message on that.