I think that construction toys are really good value. They develop a range of skills, work for both early years (3 to 5 yrs old) and later years (6 to 11 yrs old) and they last for a long time. Zoob, Quercetti, and polydron toys can be used for learning through play in early years and in later years they develop discover and thinking skills. Quercetti skyrail roller coaster and Georello for example explain and bring physics and mechanics to life in later years. 

These are less well known brands but they came out top in our testing with children and parents. You may not be familiar with them but they are widely used in schools. Polydron maxi brings shapes and maths alive at both an intuitive and easy to understand level.