Difficult to say just from your pics.
My thoughts are one of the many ornamental prunus trees, a hornbeam, or an alder.
I presume its deciduous.
Are the leaves shiny, smooth, rough, soft ?
Do the leave have a flat margin (ie lay perfectly flat on a flat surface), or an undulating margin (edges rise and fall when on a flat surface ?
Are the underside of the leaves pale or white, are they hairy?
Are the leaf stalks in pairs, or do the alternate along the branch ?
Did it flower? Was it in late March/early April ?
Are there any 'fruits' (tiny apples or plum shapes) appearing where the flowers were?
Does it have 'fruit' or catkins ?
Do the leaves colour in autumn?