All of the leading law firms offer placements that vary between 1-4weeks in length at both Easter, summer and christmas. Most are 2 weeks. They are aimed at penultimate year law students or final year non-law students. These placements are used by many as the basis of their graduate recruitment the following year. You should therefore look to secure a number of these placements if possible.
Be aware that many law firms employ critical thinking tests as part of their recruitment process.
A popular one is the Watson Glaser. Practice these tests HERE