Mozart's going thru a UTI right now, so I sympathize, especially if you can't spot the invalid yet. Baytril is what we're treating it with, and I think it's a common one used. Check that it hasn't expired, and make sure you find and bring the kitty in when your vet returns, since I don't know if it treats crystals.
If a vet goes out of town, usually he/she has someone cover for them? Then you can have the kitty seen sooner, and the correct meds prescribed. Good luck, keep us posted.