Veggie wise, you need to look at feeding 1 'cup' of veg a day per pig.
Here is a good list, it includes all the high vitamin c foods. What you want to be promoting in your guinea pigs diet is vitamin c as this is the reason behind the fresh food, as they cannot produce vitamin c themselves.
http://www.theguineapigforum.co.uk/showthread.php?42-Guinea-Pig-Shopping-List
Although there are alot of veg in the low vitamin c bit there is no reason why you can’t feed this to them, it’s nice to give them a treat of something different every now and then.
I have always given lettuce with my veg bowl as its one of my pigs favourite foods but it’s down to preference i suppose. It tends to contain, yellow pepper, the odd baby tomato (once every couple of days), lettuce, cucumber, celery and a bit of carrot every now and then. But obviously i rarely feed all of this in one sitting, it just tends to be what my pigs enjoy the most and i frequently buy more of.
When your trying out new foods, don’t try too many at once as it can result in colic or other medical problems. Slowly introduce new foods, and if you can find out from where you purchased/adopted your pigs, what veggies they’re used to it can be helpful
Another thing you need to look at when feeding your guinea pigs is the calcium and sugar levels, as some foods are better fed less frequently than others, this list is colour co-ordinated to show the veggies you shouldn’t be feeding as often, and the ones that are fine to use daily.
http://www.guineapigcages.com/forum/showthread.php?t=22156
Also don’t feed too many wattery vegetables such as cucumber as this can lead to diarrhoea.
I personally go shopping every week, the problem with fresh veg is it’s low life span, although if your keen at gardening you can plant your own veg (may take a while haha) or get the ‘living lettuce’ which my pigs love, which you can then water and it sometimes lasts a bit longer depending on when you buy it. Different shops get stock on different days, so if you can find out which day the freshest veg comes in you’ll be able to buy the stock with the longest sell by dates etc.
