I'm involved in a pre-school co-op with my 3 year old, and I also did one previously for my oldest son. I try to stay away from too many worksheets. A few worksheets here and there can be helpful, but children learn so much better with hands-on learning. I really like two websites that have wonderful, free ideas: www.first-school.wm and www.perpetualpreschool.com.
Our current group has 4 kids and meets for 3 hours, 3 days a week. They start off with calendar and circle time (pledge of allegiance, update calendar, weather chart, and introduce topic and concepts for the day), about 30 minutes. Then we have about 15 minutes to work on writing (practice name and letter and number for the day) and other skills (cutting, gluing, coloring, etc). The next 45 minutes they do projects and activities (science, arts and crafts, music, physical activity, etc) based on the weekly theme (this is where the websites come in handy!). The kids then have 30-45 minutes for snack and playtime. They need the break! For the last 45 minutes-1hour we look at books about our topic to learn more, sing songs, and read some fun books based on our theme. Sometimes the kids need some direction during their free play time, so we have games on-hand, like color/shape bingo, chutes and ladders, sorry!, etc.
If you're working with just 1 child, you won't need as much time.
Lots of fun valentine's day ideas on the websites =)