We use molecular dynamics simulations to study a semidilute, unentangled polymer solution containing well-dispersed, weakly attractive nanoparticles (NP) of size (sigma(N)) smaller than the polymer radius of gyration R-g. We find that if sigma(N), is larger than the monomer size the polymers swell, while smaller NPs cause chain contraction. The diffusion coefficient of polymer chains (D-P) and NPs (D-N) decreases if the volume fraction phi(N) is increased. The decrease of D-p can be well described in terms of a dynamic confinement parameter, while D-N shows a more complex dependence on sigma(N), which results from an interplay between energetic and entropic effects. When phi(N) exceeds a sigma(N)-dependent value, the NPs are no longer well dispersed and D-N and D-p increase if phi(N) is increased.