It is strange that according to this site Bolgar is one of the least visited WHS in Europe, as the place is very popular during summertime. Probably the reason is that most users are from Western Europe and few of them explore Russia, especially places distant from Moscow or Saint Petersburg.
Bolgar is the most significant remnant of Volga-Bulgarian Kingdom, ruling this part of Europe from X to XIII century. Bolgar was the capital of that kingdom and the place where they took Islam as their religion. It seems that current policy of Tatarstan is to praise local and Islamic elements, so the site was recently largely renovated, however excavation works are not finished yet.
There are many well-preserved buildings in Bolgar, such as several mosques and mausoleums and remainings of Khan’s Palace. The site is quite large and to visit all inscribed sites it is necessary to have at least 2-3 hours. You may cut this time significantly by renting a bike (100 rubles per hour) which I did. The whole site is very nice and the view on Volga river banks is really stunning.
Bolgar is located about 180km from Kazan, but getting there during summertime is not an easy task. I tried the speedboat (once per day) but the tickets were sold out for the next five days. Bus tickets from the central station (near the river boat station) were sold up to three days. Fortunately, there was one remaining ticket for that day from south bus station (Yuzhnyi Avtovokzal) located in the southern suburbs of Kazan. I strongly suggest booking the tickets in advance, at least during summer. Rented car is also an option (be not afraid of Tatarstani roads, they are good, but beware of police frequently controlling speed) and even without GPS getting there should be easy - road signs to Bolgar are numerous. There is also possibility to get there from Ulyanovsk, located about 130 km south of Bolgar.