From the hotel walk directly out the door, which points you toward the river. Couldn't be easier. Head toward the river and the London Eye (the, um, slightly oversized ferris wheel – hard to miss). Instead of crossing the river, turn right and just walk along the waterfront pathway, Take in the sights on your left across the river (your mapping app will identify them all) and also note the various restaurants, pubs, bars, coffee places, food carts etc. on your side of the Thames, on your right. Keep going until you feel like it's time to turn back.
Do this as soon as possible after arrival, and you will know you have a lovely walk dialed in and ready for any time you feel like a break and maybe some nearby refreshment.
If you make it as far as the Shard (look up and ask yourself "which tall, highly visible building got nicknamed the Shard, I wonder?) then you have reached London Bridge. From here you can take the Tube back to Waterloo station and walk from there to the hotel, you can cross London Bridge and explore, or you can go a little further to Tower Bridge, cross there, and take in some very historic sights.