2 1/2 year old spent 4 hours outside yesterday. Today has 102 fever. No other symptoms. Could this be from heat? Should we go to urgent care?

Illness. Being outdoors in the heat is not why your 21/2 yr old has fever. It is likely starting an illness. Call your doctor or have him seen. Not an emergency.
Fever. Does the child have any symptoms of ear/nose/throat/respiratory infection? Is (s)he sweating, or hot & dry? If hot & dry & no signs of illness, cool down with luke warm/cool bath; give children's advil (ibuprofen) (dose according to weight & age) & cool fluids to prevent dehydration. If temp doesn't fall or continues to rise, wrap in cold, wet towels/sheets & take to er.