Young adult is such a broad term... some 50 yo's consider themselves to be young adults.
Couple of random thoughts:
College age is different than young adult... trying to mix these groups is often hard because life exp's are worlds apart, and the two groups have diff thoughts as to what fun is =) I was in a very active college group, and we always knew when someone was growing into the young adult group because they wouldnt be interested in going out every night, spending time with tons of people all the time, etc etc. They enjoyed staying home, smaller groups, etc. We used to call them fuddy-duddies until we reached that age. Then we understood =) If you want to design a college program, the coffee-house worship environment is really popular and pretty successful. College age students usually love a great praise and worship exp, with socalization, food, and a dynamic Bible study experience. they also want OWNERSHIP. You can design and plan things all day long, but they won't buy into anything you do unless THEY are planning and hosting the things... We had a college minister come in and take over things, and he lost 80% of his program because the students had no ownership anymore. He didn't value their opinions and priorities. THEY know what htey want. If you want to target this age, grab your most faithful attenders and get them involved in planning things... if they're not already.
Same goes for young adults - they want ownership. They dont want someone always planning their activities for them. Young adults, in my mind, are post college - young 30's. Many are newly married, starting families, and some are single and career minded. Thats a big group! Its important to find a common ground for them to meet... you can say their faith is common ground, and that works for awhile, but at a point they need something more than their relationship with God to help them bond. Small group BIble studies are great - we really enjoy those and they are very successful. Find a couple of leaders to moderate and lead discussion, choose a good study, plan it for a home environment, and provide light food and drinks - people usually come. Be sure to keep it relatively small (less then 12 usually), have time for socializing, and let the group take it where they want it. We are in a small group now and we plan social activities and charity activities on our own... the only thing the church "programmed" was the leader, place and time, and studybook to get us started. We were turned loose and have been growing ever since.
Ownership is key. Its diff to provide activities for children, but adults want to be a part of things. Every group of people are diff and just because the church up the road is having success with a coffeehouse environment, doesnt necessarily mean y'alls will... your church may not be interested in that. Meet the people where they're at. And also define WHAT age you're targeting... young adults is too broad, you need to figure out if you're thinking college age, single adults, young families, early thirties, etc. Focus on one group at a time instead of taking on an entire decade or two =)
Good luck!