Je pouvais'aurais pu faire beaucoup de bonnes choses dans le passé.

If you want to express regrets, you can use "j'aurais pu", if you want to say itneutrally you can keep "je pouvais" .
what does "bonnes choses" stand for?
if it means being nice to others you use "bonnes actions"
if it means having fun, you must rephrase "j'avais l'occasion de faire plein de chose qui m'amusaient"
If it means having great projects you can no longer pursue you say "j'aurais pu entreprendre tellement de choses"
If it is neutral (no regret) it's more like:
"j'avais l'occasion d'entreprendre tellement de choses"
I don't know if many french people would use "dans le passé". We may refer as a precise time by saying "à l'époque".