Today is my guinea pig's birthday as well! I made him a guinea-safe cake, after finding a recipe online. I started with taking some natural timothy hay pellets, (your guinea pig's usual kibble will also work) and ground them into a finer powder, then I poured it into a small bowl. I added oat flour to the bowl and mixed it, then added water until it formed a doughy substance, after I added a very small amount of honey along with a very small amount of vegetable oil. I took the dough and formed it into a cookie sort of shape, I baked it at 350 degrees for around ten minutes. If you wanted a harder, more biscuit-like cake you could go for 11.5 minutes and if you want a softer more doughy cake you could go for nine minutes. Happy birthday to your piggie!