Don't buy anything to eat or drink in the vicinity of the entrance to the Tsaravets Fortress, they charge 3X the usual rates for everything.
Unique Suggestions: Ask the price in advance and then decide if you *really* need to buy it...
Fun Alternatives: There is no shade on the hill and it can get quite warm.
Bring water and snacks with - it can get quite warm and water is recommended.
you can do an organized tour to the nearby monasteries. contact the Balkantourist for that. But it is very expensive. 80 lv. You better hire a cab which charges a little to get you to several monasteries and buy the > Turnovo monasteries: A Guide for 5 lv at TIC
I absolutely recommend when jumping in a taxi to try to communicate with the taxi driver. Having heard reports that all the taxi drivers are very fair (you can't say this in Eastern Europe) I tried it myself. Therefore:)
One way from my hotel to Tsarevets fortress - 1.07 LEVA
The return - 5 LEVA (after trying to go for 5 EURO)
Unique Suggestions: Negotiate the fare first if he won't have a running meter. It's a must.