Credit card bonuses are your friend.
Flights, hotels, and trains can be had for free or a low price.
Does this require a certain amount of spend in a set amount of time though?
Yep. The standard is around $3000/3 months.
That can be tough for those who spend little. We time it for large bills (our house and car insurance take cards) and buy grocery gift cards if it's getting close. There's an advanced technique that "manufactures" spending, too.
There are a number of cards that rebate $150-200 for $500-1000 spend. Get a few of those and you'll soon have $4-600 for a trip. If your SO plays, you could get $1000 for a trip pretty easily.
However, there's an order to getting cards due to bank restrictions. If you can manage the minimum spend, you should start with the Chase non-branded cards.