@OneHornedFlyingPurplePeopleEater decent bikes. We had a second hand one a couple of years ago that is played with every day and they are SO much better than cheaper ones. Try Isla bikes or frog bikes and you can get spare parts to hand them down or resell.
We also loved magnet tiles but again buy good quality or the magnets fall out.
Days out are popular too. Get the grandparents to take them and go out for lunch yourself!