The San Diego County Fair is running through July 4th
http://www.sdfair.com/ The fair is also going to affect your drive heading up or down the 5 into San Diego proper. The freeway gets congested because of the additional fair traffic, so plan your trip for off-times - maybe after 9:30 in the morning, and returning after 6:30 in the evening. Or better yet, just take the train.
I second the Midway. Excellent tours. Within walking distance is my favorite hole-in-the-wall mexican food place - Mexican Fiesta.
http://www.yelp.com/biz/mexican-fiesta-san-diego Read the reviews for tips - there is no onsite seating (but there is public seating areas across the street) and they are cash only, but imho eating here is a fun San Diego thing to do.