Have them take Norway from Denmark in the 1668-peace(when they took Copenhagen)
There was an identical thread a while ago but I can't seem to find it. Sweden's biggest problem is its small population. They'll just never have the numbers to equal the other players in Europe so they need allies. I think the best possibility is to prevent the split between the Swedish and Polish branches of the house of Vasa or at least leave them on friendly terms.
A strong Swedish-Polish relationship could push back Russia and keep it from ever reaching the sea.
perhaps my previous threads can be a help of sorts.
AHC: Powerful Swedish Navy
WI: Gustav III longer life