Aira Force, Ullswater
These are on my list of all-time favourite falls but are easy to get to. To the north of Ullswater, where the A5091 meets the A592, simply park at the National Trust car park and head up the relatively easy paths to get to the higher and lower bridges that cross the falls and provide spectacular viewpoints.
Stock Ghyll Force, Ambleside
Stock Ghyll runs through the middle of Ambleside and it's easy to get to the falls - start by taking the road behind and to the right of the Salutation Inn. For the best mix of views walk all the way to the top bridge, go across, head back down to the lower bridge, and then return to Ambleside for a well-deserved coffee and cake.
Fairy Pools, Skye
Although there are concerns about the number of visitors to this one I still think it's worth making the trip if you're on Skye - you can get a bus from Portree and I'd recommend walking as far as you can along the series of falls as the closer you get to the mountains the more spectacular things get (having walked from the other side, this was where I started!).
Four waterfalls walk, Bannau Brycheiniog
Head to Ystradfellte, Powys, in South Wales and park at Cwm Porth or Fwaun Hepste. From here you have options on this ever-popular walk that's dog and child-friendly (with care).
Nant Gwryd, Eryri
When anywhere near Yr Wyddfa (Snowdon) you should visit the Watkins Falls heading up to the summit from the south. My suggestion is also, while you're on that side of the mountain, to head east a little and visit these falls behind the Pen-y-Gwryd Hotel (at the junction of the A4086 and A498).
Skelwith Force, Elterwater
These falls on the River Brathay, as it flows from Elterwater to Windermere, are always worth a visit, especially after heavy rain. You can get the bus from Ambleside or just park at Skelwith Bridge. For a longer, but still gentle walk, keep going to Elterwater and walk between the two.