If you have extra time, you might consider hopping to and shopping through Covent Garden Market that offers an antique collection of variety stores, including shops selling unique British gifts and handmade crafts.
If you'd instead want to continue the party, you might head to a late-night club like Fabric or XOYO or take a scenic walk around Tower Bridge. Moreover, if you have less time, you can swap indoor karting with visiting Madame Tussauds wax figure museum or opt for Indian cuisine lunch at Masala Zone.