You are really quite early in the year for the best scenery. I suspect the US is quite a bit colder than wherever you are from - but typically the trees will only be coming into leaf at the beginning of May - and you will probably not see all of the flowers you expect. While you can can daytime highs that are quite warm, you can also still get evening lows that are quite chilly.

Thank you nytraveler - any thoughts on the best route though? Whilst the scenery still won't be at it's best I imagine it is still quite different from what I'm used to......

Is your rental car company OK with having their US rental car being driven in Canada and turning it in in Rhode Island?
Make sure your insurance is good for US and Canada. If you wanted to travel south of Lake Erie instead of through Canada, you could take the Lake Shore Limited out of Chicago at 9:20PM any night.

Danielle... i know exactly what you are seeking...

if you are not in a hurry, the BEST, most scenic route is RT 20 across New York State and into New England... the route starts south of Buffalo and passes through farmland, villages, antique shops, and along the top of the beautiful Finger Lakes... there are amazing stops along the way and a taste of local diners and small businesses... if you wish to veer off rt20, you can visit some Finger Lakes wineries...

If you choose to stay on the NY State Thruway(RT 90) you will see little other than cars and concrete bridges, however you can rejoin 90 at any point as it runs parallel to Rt 20... ..

A great place to stop for the night is the Albany area as it would put you more than half way and allow you to choose to rejoin the highway or continue on the backroads...

There is nothing you won't see on Rt 20 that makes going through Binghamton a better choice ... good luck

I would agree with Gary, but pick up Route 20 off Exit 48A of Route 90 (go South on Route 77 to Route 20). Skip the city, strip malls, and about 100 traffic lights.

Stop in Skaneateles or Cazenovia for lunch depending on what time you get up and leave the Falls. Take a side trip down Route 14 for 15 miles or so driving along the lake and view some of the vineyards. Also beautiful homes along Route 14 just South of Geneva along the lake.

You got two excellent route suggestions, from garyt22 and DaveS. I have drive this route (actually both of them) and it is very scenic.

But it will do you no good at all if you cannot give a positive answer to tomfuller's question. You will need the consent of the rental car company to drive it from Canada and leave it in the US. You very well may find it easier to cross the border and then rent.

Finally, you may well get frost in the mornings if you are traveling in early May, and some tourist locations may not be open until after Memorial Day, the last Monday of the month.

Thank you all for taking out the time to reply. Due to time constraints we've decided to veto a lot of the driving we'd planned and just focus on flying from San Francisco directly to New York, then Rhode Island and we're taking the train to Washington for a few days. We'll keep these excellent suggestions for another (longer) trip in the future!