well i do not know much but there is bhavnath which is the foothill of the majestic Girnar mountain. lots of devotional places. You cannot climb the Girnar mountain as you would not be having that much time but it is once in a life time experience climbing that majestic mountain.
then you have uppar kot, which is the magnificent fort of Junagadh. Adi Kadi Vaav and Navghan Kuvo is another famous tourist spot. all these are historical places if you are interested in them.
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have the Zoo of Junagadh - sukkar baug which has a good collection of animals including the famous asiatic lion, of course. Then may be some museum of Nawab Mohabbat khan, who was the richest nawab of his era. Lot of places of touriist interest.
i would suggest if you are two or three people, hire a auto and tell the driver to take you to all these places. it is a small city and you can do all these places in 4 - 5 hours.
and do not forget to taste the local kathiyawadi thaali. Geeta lodge near the railway station is famous for it. they serve lip smacking thaali in afternoon till 2 pm, if i rightly remember. and in the evening visit Kalwa chowk and you can have a range of street food there.