You can get cash from any war by sieging holdings (once you take the holding, you gain an amount of gold that depends on the holding type and level). If you use only your vassals´ levies, you can get that cash without any expenses.
If you want to gain prestige without gaining any new title, there are some options:
- Excommunicated ruler CB makes said ruler give up his/her title to the designated heir, as well as gives you pretige and piety.
- You can join any war and participate as much as you can. If your side wins, the prestige is distributed among the allies taking their contribution into account.
- You can use a "Claim x title" CB for one of your courtiers that isn´t part of your dinasty. The corresponding title will go to your courtier and you will get prestige for winning the war.