One visiting the Anaheim area would love to explore nearby Brea. By car, just head north on the 57 freeway, via the 5 or 22 fwys in Anaheim/Orange area. Depending on teh time of day, the trip is only about 5-10 min.
If traveling by bus, Brea is very reachable. From the Disneyland area (catch at Katella Ave/ Anaheim St., you can take the 47 heading north to Brea Mall (look for the bus head sign to say "Brea Mall". If it says "Fullerton Transportation center", you'll have to wait at the center and wait for another 47 bus having the "Brea Mall" headsign). The time to reach Brea fom Anaheim is about 30 min if heading on the route that goes straight to Brea Mall. Check website for route schedule.
When travelling by bus in north Orange county, a good place to launch your trip is in Brea, at the mall's popular transit loop. Several OCTA buses come and depart here, as well as a useful Metro LA county bus that heads off to Brea's northern neighbor Pomona. The loop is located off State College Blvd, between Imperial and Brea Blvd, and is located by Reb Robin restaurant. A popular bus to take from there is the 29 which goes to Knotts Berry Farm and all the way to the shores of Huntington Beach. Also, the 47 bus goes all the way to another beach area, Newport's Balboa.