sagrada famiglia and pare guell

That would be a very large gap. Unless you're really unlucky with the weather, you can kill some time wandering back toward the center from Parc Guell on foot. Use some additional time looking for other modernista buildings once you're in the Eixample, though that will probably involve some overshooting of La Sagrada Familia. A lot of maps of Barcelona have some modernista buildings marked--though typically not defined as such.

Another thing you can do is hit the very pretty Sant Pau Modernista Site. It's a place where you can walk right up and buy a ticket for immediate entry. It's fairly near La Sagrada Familia. You'll be looking for food at some point, too.

In Barcelona, you can never get bored, it's always easy to assemble something to do/see.

First things first though: please do make the effort to correctly write the names of places. Two reasons: if you search online you won't find anything if you don't type them correctly (computers do not guess) and then, it's deeply annoying as a local since it seems as if "you're not bothered" to even try to respect Catalan, our language. Yes, ok, some cultures might be pickier than others when it comes to the last one, but that's the way it is.

Anyway,

If you're going to Park Güell in the morning, 8ish or so, you'll probably be done by 10ish (give or take, it depends on how much you like art and architecture of course). If walking is not a problem, moreover taking into account you're going downhill from there, I'd suggest visiting nearby Turó de la Rovira (aka "bunkers del Carmel") where you'll find remains of one of the anti-aircraft batteries from the Civil War in Barcelona (1936-39) set up to defend the city from the fascist air attacks from the Spanish dictator Franco as well as his allies Hitler and Mussolini. The city saw some of the worst atrocities to civil population up to that date (pre-WWII)

Note though that the reason for visiting these days are the views over the city since, as said, there are few (if any) remains of the barracks and there are no longer any cannons. I will reluctantly point you to a website with some more info about this, but note, as mentioned earlier, there have NEVER been "bunkers" as such but a defensive position with open-air cannons. But again, popularly this site is known as the "bunkers del Carmel" and some bloggers seem to fall foul to this myth.

Also, the neighbourhoods of the district of Gràcia at its feet can be a pleasant walk. Gràcia, founded in the 17th century, was an independent town in the plain of Barcelona until the late 19th century when the growing city "absorbed" it -same than many other towns and villages that are now simply neighbourhoods of the city. It's full of low-raised two to three-storey buildings, tree-lined squares with terraces and a rather bohemian way of life. It's one of the few places in Barcelona when one can still feel "as if in a small town" yet being within a large city. I wrote a post about this, check WED16 out here.

From there you can then have lunch somewhere (some ideas here) (+) and walk to Sagrada Família along Diagonal Avenue, where you'll be able to admire some very fine Modernist architecture, such as Casa Comalat, Casa de les Puntxes, Palau del Baró de Quadres, Casa Macaya, etc.

I also second Ann's suggestion above regarding Sant Pau Recinte Modernista, at 15' walk from Sagrada Família and it's one of the gems of the city.

Enjoy!

(+) Note Turó de la Rovira is to the north-east of Park Güell, so you might want to go there after the visit to the park and then proceed south to Gràcia.