When you first arrive..

I agree with BG, your purse or daypack can get awfully heavy if you don't limit what you put in it. I try to pack my purse/daypack as lightly as possible as it may not feel light when you first set out but after a few hours, it can get really heavy. Sunglasses, camera, guidebook, map, small journal. Sometimes a tiny umbrella. I don't usually carry water or food.

What I need on the plane and what I take in my day pack are nearly identical. I keep my firstaid kit, medications, sewing kit, umbrella and/or or jacket, documents, etc. in my day pack at all times, along with wipes, kleenex,ziploc bags, nail clippers, pen and notepad. On the plane I also keep my sketch supplies and any reading materials in it - those will probably get moved to the suitcase before heading out for the day. If it doesn't look like rain, the umbrella may get left behind, too.

I should probably clarify, I meant to focus more on the packing aspect, not so much what to do during the day. I will look around to see if someone asked that question, thanks!

My daypack doubles as my carry-on (yes, I check my luggage). I usually have to do a bit of shifting things around, but not much. I'll already have snacks and water in my carry-on, along with my guidebook and camera. I also travel with a change of clothes and a few toiletries, so I'll stick those in my luggage, along with whatever other book or magazine I brought on the plane.

I don't like to lug a lot of items around in a daypack or purse in any case, but especially after a transatlantic flight. I think all you need for the first day is a guidebook, camera, city map, sunglasses, perhaps a scarf or umbrella, depending on the weather, in addition to any meds you may need.
This is assuming you have your passport, ATM and credit cards and most of your money in a moneybelt -- and you are wearing comfortable walking shoes!

I also follow mostly what Grier and BG do. I have two personal items on the plane: the bag I take around with me during the day (i.e., my messenger bag) and my luggage bag. In the day bag, I have a guidebook, itinerary, iPod, water bottle, camera, little umbrella, sunglasses, etc. (pretty much things that you carry around with you during the day). The passports, money related items, and other important documents are in the money belt. My toiletries are easily accessible by being on top of everything in my luggage bag. When I land (or even on the plane), I clean up by quickly accessing my toiletries, check my luggage bag at my hotel, and then walk off with my day bag that already is packed with the items I need during the day.

I usually use my daypack as my go-to pack for the plane ride. Normally, I can stuff it inside my larger backpack, but I put the larger bag in the bin overhead and keep what I want on the plane--books, maybe travel scrabble, etc--within reach in the daypack. This is different than what I would want to carry around while sightseeing.

I would suggest you use a packing cube/stuff sack/ziplock bag/whatever to contain what you think you will need while sightseeing and pack it in the top of your bag, or in a side pouch if you have one. Then, once you de-board the plane, you can pull out your stuff bag of sightseeing necessities and put that in your daybag, and use the space now created in your larger bag to store the stuff you used on the plane.

If possible, I like to check into my hotel and leave my bag there, and then begin sightseeing. Some hotels allow you to drop your bag off early even if its too early to check in, so you might want to see if you can do that after arriving in your destination. Finding my hotel helps me feel oriented, and plus I don't have to go back somewhere to pick up my bag.

There are some good posts on the boar dabout what people carry in their daypacks if you'd like some insight.

My last sentence of the post above was supposed to be:

There are some good posts on the board about what people carry in their daypacks if you'd like some insight.

Aargh, typos.

I second the idea of packing cubes. With the 8kg limit its amazing how quick a carry on is filled up with not alot of extras. Plus in the event you have to check - its goto to have one light change of clothes to stuff in your day-pack-personal item. The cubes make it easy to move in the bag as you drop it off at your hotel and go go go!

We always arrange to drop our bags at our hotel first thing. We have never been refused the ability to do so, and it is so very much the worth to know where your hotel is and that there are no issues. From there, we quickly repack a single day bag (maps, cameras, guide, valuables, etc) and go. That gives us until late afternoon (or later) to hit the hotel, adjust, and hit an evening meal.

We are actually staying in Leicester with family who will be picking us up later, so we don't have the option of dropping them at a hotel. I'm not looking forward to using left luggage..

If you are going to Leicester later, then where are you storing your bags when you arrive? At the airport?
Then are you going all the way into London, then back out to the airport for pickup by your family?
Sounds a bit tiring to me after a long flight.
Have fun whatever you do!

It will be a long day! I think we are going to leave them at the train station in the city. She works right outside the city so she's going to come pick us up after work (although we haven't arranged where yet).