This looks a lovely trip in March/April time. I'm heading to Croatia in September myself and plan on doing a similar trip. I'm not going to lie and say that you can get this trip cheaper by picking different accommodation but I've picked some lovely apartments, B&B and hotels and I've checked all reviews I can find and they are all good so you should feel confident in the choice of accommodation I've picked, however you can change them if you want to save some £ but I thought I'd pick some more luxurious places to stay.
Flights are to/from Manchester and include hand luggage only.
You'll collect your hire car after arriving around 9:30pm. You'll head straight to Zadar and stay for 1 night in the lovely Apartments Petra which have great reviews.
I'd suggest after a leisurely wake up you drive to Plitvice Lakes. I couldn't resist putting in an overnight stay at Plitvice Lakes, this way if the weather is not co-operating you have an opportunity to do some more walking and hikes the following day. The accommodation I've chosen is the Tourist Grabovak which again have really good reviews.
30th March-1st April
You'll drive to Split on 30th, you will spend 2 nights in Split so there is no rush to leave Plitvice Lakes. The accommodation I've picked is Sandra House which has again amazing reviews. Spending on how much you want to see and do you could visit Krka National Park or even take the ferry to Hvar if this interests you.
After leaving Split you'll drive to Dubrovnik. Again the apartments I've picked here are the Green Park, lovely self contained apartments that are ideally placed for you to explore.
You'll spend a final night in Split, half way between Dubrovnik and Zadar. As you don't need to be at the airport until late (car hire due back 8pm) you can choose how you split up your time on your final day. I've picked the Peruzovic which is the no1 speciality lodging on Tripadvisor so you have a really lovely place to spend your final night.
2 return flights to Zadar from Manchester, book with Ryanair £73.96
1 night in Zadar £24.47 book at Ebookers
1 night in Plitvice Lakes £35.24 book at Ebookers
2 nights in Split £59.30 book at Ebookers
2 nights Dubrovnik £78.08 book at Ebookers
1 night in Split £29.40 book at Ebookers
Car hire book at Car del Mar £21.16 - I'd suggest using a company such as Insurance4carhire which cover the excess if you want additional cover.
Also remember some places charge city tax so this will be extra.
Ebookers discount codes can be found on HukD voucher forum.