Raids where if your team dies, you will fail. Then, add in an almost unstoppable number of badies, then say, Go into the base, get some object, and get out before you die. You can go for all stealth, fast and hard, or go real slow and try to mow down everyone. Even better, add in more then one fireteam, maybe three, and have some people go stealthy, some go loud, some cause distractions and diversions. Basically anything that gives the players a lot of tactical control over the situation, letting them do it however they want.