Sunny Beach is bit “lively” and overpriced – purpose built for the Tourists.
However, there is a very good restaurant called Djanii’s (or something) which is worth at least one visit.
Change your money there.
No need for a car in my opinion. You can easily get around on foot as Sunny Beach is really just one long, narrow beach strip.
Nessebar lies to the south of Sunny Beach and is definitely worth a visit as the old town is a UNESCO site and very interesting.
Should be hot too.