。
She used to play tricks on me a lot.
Literal
[topic-は] often I [to-に] mischief [object-を] did [used-to-ものだった].
いたずらをする ('to play a trick, do mischief') is the standard collocation, with に marking the target/victim of the mischief. The vivid past-recollection ending ~たものだった adds a wistful, looking-back-fondly tone — the speaker recalls these tricks now with affection rather than frustration. いたずら is itself a noun spanning everything from playful pranks to genuinely naughty behavior; context picks the severity.