I mean, training a new position or special ability requires rest packs. Either directly or indirectly. Training the whole squad 25x with maximum set of drills as well. Playing friendlies and winning could work if you're the away team as you wouldn't waste condition (wouldn't gain any packs either on the other hand, but it's just a rest pack, the other i could live without as I have a surplus already).
And other activities seem far fetched, like winning auctions with 5 bidders. I'm not interested in expanding the squad. Another one is assigning personal trainers to players or general trainers to youths which require tokens and/or rest packs.
So investing rest packs (in)directly and cash.
As for the extra players, that was exactly what I intended to do: sell them. Unless I would get a 8-9* striker.