Here are a few on offer from Balkan Holidays via your rep.

Lazy Sunday Fishing etc adults 55 lev child 33 lev
nessebar...40 lev adults child 24 lev.
balkan fiesta 55lev adults child 33 lev
country and people 78 lev adult child 47 lev
balchik and kaliakra 45 lev adult child 27 lev
jeep safari 85 lev adult child 51 lev
dolphin show 35 lev adult child 21 lev
pirates show 55 lev adults child 33 lev

...while the street traders do offer a cheaper price beware cos our Lazy Sunday trip was cancelled by the boat operator without informing us. The weather was good and we were on time, we were offered the following day but I declined....We were lucky enough to spot our rep who saw us waiting at the pick up point half an hour after the designated time. She smoothed things out and we got a full refund. We might not have been so lucky if a street vendor had been been our choice.