For various reasons I've not been into Central London for several years, but I have a trip coming up - an incredibly rare day off from DS. I'll be arriving about midday, and will have about 7 hours until I'm meeting friends for dinner. I've booked the National Gallery, and I'll do a bit of shopping on Oxford Street as I have some vouchers to spend and don't often get a chance. I'll probably not use the tube so will be on foot in that area, and will enjoy wandering through St James' Park and around the streets.
Any other suggestions for things I should do? Preferably cheap/free as I don't have a big budget to spend and will need to save it for dinner!