My guinea pigs seem to enjoy chewing sticks, and this is supposed to be good for their teeth too. You can buy willow sticks in pet shops, or if you have access to an apple tree (not treated with any chemicals such as pesticides) you could just snap off a couple of small branches to give to them. Some other types of trees are suitable as well, but you'd need to do a bit of research to make sure you avoid any poisonous ones.
I find that attaching a short stick diagonally across the corner of a cage is good because it helps the guineas get a grip on it - use a bit of wire to attach both ends of the stick to the bars of the cage and tuck the ends of the wire on the outside of the cage, out of their way. I tried using string but it got chewed through instantly!
Another thing you can get in pet shops are willow balls that can be chewed and pushed around, and I think I've also seen some that come with a treat inside to encourage the guinea pigs to investigate, so you could try something like this.
Good luck and I hope you find lots of things they enjoy!