Cats, Dogs: both have curious streaks, and both are social.* Neither have tool use, but both adapt well to interspecies communication.
Cetaceans: Arguments over whether or not they are sentient are already present in the literature. Their lack of manipulators is, in fiction, often solved with waldos.
Primates: Some already are indisputably sentient (us); others are damned close if not already (most other apes, some monkeys), and many are VERY smart animals. All primates are highly curious.
Raccoons, otters: Argument made above. Increasing brainmass and better thumbs will make them MUCH more capable. They both make primate curiousity look rather mild.
Ferrets and other Weasels: in addition to being as curious as Racoons, they are also far harder to intimidate... Fearless, into everything, and viciously competent predators. The stories of wolverines and tormenting hunters are legion and many are credible.
Birds in general: Pick any large body bird. Social, vocal, and capable of tool use already, and many species already capable of conceptual language use.
*Cats sociability is dependent upon densities... but even the notably singular tigers and bobcats will, when constrained and well fed, develop prides just like lions, wildcats, and domestic cats.